Email Address associated with my AppleID is different in iOS devices (under "Advanced" heading) than iCloud.com?? This "Advanced Email Address" displayed on iOS devices IS NOT my primary iCloud email address!

What is the "Advanced-->Mail" (email) address within iCloud settings under "Account" on iOS devices? My iCloud account settings on all iOS devices (iPad 5.1.1, iPhone6 8.3, and iPad2 8.2) have a different email address listed under the Advanced heading than my primary iCloud email address? I'm sure it's somehow associated with an old iCloud account of someone in my family (took us a while to each break free with our own iCloud accounts in the early iOS days)... When I log into iCloud.com on PC I only see 1 option that specifies both AppleID and Primary Email Address (of which is correct and aligns with my pAppleID on iOS devoices), I see "Alternate Email Addresses" of which I have none listed (rightly so)... BUT, I see no option in iCloud.com settings for having my email address be anything different than my AppleID??? How do I change this "Advanced" email address for my iCloud account on my iOS devices?