How are Game Center “Friend Recommendations” people I know in real life? (Meetup/Facebook data?)

This seems like a major privacy concern, but mainly I'm just very curious how this can possibly happen:

The Game Center app suggests primarily people I have met in real life... but have had NO online interaction with!

I life in a high-population US city, yet at least 2/3 of my Friend Recommendations (maybe more if they had photos or knew these peoples' last names) are people I have actually MET in person and know vaguely... but who I do now know well and who are NOT friends/contacts in any online service.

• I have never emailed them (with two exceptions); we don't even know each others' email addresses, and even if we did, my Apple ID email is not one I use for actual emailing purposes. Nor do we know each others' phone numbers, nor gaming nicknames; often not even last names. I didn't even know that they owned iOS devices or played games at all!
• We have certainly never played games with one another before; in fact, I have never used Game Center's friend feature and I don't have any GC friends. I only play random matches, and almost always on non-GC game services, for that matter.
• None of them has ever made a friend request to me. I've only ever gotten one GC request, long ago, and I denied it; it was not someone I knew.
• Some of them list "zero games in common" so it's not even that (and anyway, millions of people have games in common with me).
• Only one of them is in my Contacts (iOS/Mac address book), and knowing how serious they are about privacy, I doubt the email address I have is their AppleID. I know your address book can (for now) be accessed without permission, but these people just aren't listed.
• But I DO run into these recommendations in person every so often!
• These cannot be randomly selected by geography because a) they're not super close to me in miles, and b) there are millions of iOS users near me. No way that most of the suggestions would be people I've met, if this were chance alone. SOME source of data is being used by GC, and I'm baffled as to what it could be.

The closest three theories I can come up with are all pretty implausible:

1. Indirect Facebook connection. These people are NOT my Facebook friends (with a couple exceptions; I never actually use Facebook but I do have an account, and if someone requests, I generally accept). I double-checked that. But they MAY be "friends of friends" possibly. But how would Apple obtain my FB friend list? And if they did have it, wouldn't they just suggest my actual FB friends, not friends-of-friends? My Facebook login email address is an alumni address that I have never used for any other purpuse; it is not my Apple ID. And I have diligently set nearly every option to "hidden" on Facebook: even friends see a mostly blank profile!

2. Meetup.com. This is a 100% correlation! I'm in several local groups that use Meetup.com to schedule meetings, and ALL of these Game Center recommendations are memebers of Meetup groups that I too am a member of. That's how I've met most of them. But Meetup has no "friends" feature, and most of these people have had no online connection with me (Meetup messages etc.) through Meetup. We've just attended the same events--which do have online RSVP lists--or else we are merely listed as members of the same group(s). (But we're talking large groups, so I haven't actually met most of the members, and I have met most of these GC recommendations.) But how would Apple obtain my Meetup info? Again my Meetup account's email address is not my Apple ID, and I have never used that email address with any Apple service (although it is on my Contacts card).

3. Maybe Apple harvested my address book/contacts list (which does NOT contain these people), and also harvested the contacts lists of these people, AND also harvested the contacts lists of people we know in common who happen to have both of us listed. Then they put it all together. (That's a stretch: the people I know the best on Meetup--whose info I do have in Contacts--are Android users!) But again, why not recommend my actual contacts, instead of contacts-of-contacts? My actual contacts--which contain numerous iOS users and games--are notably absent from the recommendations (with one exception).

Obivously Apple cannot, and does not, literally know who I have met. (No foil hat for me!) But they DO seem to have some data source that correlates closely with people I have met (and Meetup activity especially). I'm dying to know how that's possible.

This is a privacy concern, for at least three reasons I can think of:

1. GC lists these people's real names AND GC nicknames together--BEFORE I have accepted them as friends! And vice versa, I'm sure my nickname is being shown with my real name to total strangers. That means strangers, employers, stalkers, whoever, can now tie a person to their nickname (often the same one they use all over the Interner, not just GC). But we're not talking random suggestions: this is being show to people I DO know. Maybe I don't want them Googling my online nicknames! Maybe I complain about my work with that nickname 🙂 Maybe I want my online friends and real-life acquaintainces kept separate. I definitely want EVERYTHING kept separate from these people I barely know but run into from time to time! One of these people is, in fact, on the sex offenders registry, and has stalked a friend of mine both online and offline. Maybe he'll learn my GC nickname--the same way it has shown me his--and can now Google that!

2. If I have a lot of "games in common" listed, then they can peg me as a "gamer," which is sometimes a negative. Maybe I want my boss (or whoever) to think I'm more serious, even if he games too, and now he knows I have 30 games and wonders why I'm not staying late to work 🙂 Maybe I just don't want to be roped into being "friends" online with my boss, or having to let him win games! Again, although this is a small issue at first glance, it's less small when things are being shared with people you really DO know in real life. What if my hypothetical stalker sees I have games in common? Sure, I can deny their friend request and cancel their 500 game attempts, but I'd rather be 100% hidden from them in the first place. I have no prior online contact, I want it to stay that way, and I've never given Apple permission, that I know of, to share my real name with people I never friended. (And even if I did, it should be with random strangers--not actual acquaintances.)

3. The big mystery--how is Apple getting data that correlates with who I've met in the first place? I'm guessing that some people see the same pattern I do, and some don't; which might lead to an answer, but so far I'm stumped. (If this does tie to Meetup some bizarre way, that would be unnerving because Meetup ties me to real places and specific times.)

Theories are welcome! And a basic question: what data sources DOES Game Center have available to use in making friend suggestions? I would have guessed maybe Location Services (nearby people) and people who have played a game with me (or made a friend request) in the past. But those sources are not what I'm seeing.

(Even weirder is that nobody else seems to have posted about this happening; it may not happen to everyone, but surely some people have had this same question? I feel like I'm in a Kafka novel...)