I've been seeing similar problems. I have three contacts with "Shannon" as a first name. When I tell Siri "call Shannon", instead of responding "which one?", she would only recognize "Brian and Shannon Smith". Even if said "call Shannon Smith" she would only find "Brian and Shannon Smith" (which didn't even have a phone number associated with it, which she tells me). Oddly, when I said "send a message to Shannon Smith" she chose Shannon Smith as expected.
I have more of a problem with businesses. Siri has a hard time with business names at times especially if the name of the business has a person's last name or words with ambiguous pronunciation or non-standard words. Another oddity here: she can't find an entry for "Menard's" but she can find it if I change the entry to "Menards".
But here's a very specific issue: I have a contact named "Bike Shop". It has always worked (even before Siri on my iPhone 4) when I would just say "call bike shop." However, since some time in August or September of this year (2013) not only does Siri not find the contact, she immediately replies "I found three bicycle shops. Tap the one you want to call" (of course the number varies dependent on my current location). The listings appear to be from Yelp and sometimes they are actually bike shops but sometimes not. I have an iPhone 5 that I have NOT upgraded to iOS 7. I'm at version 6.1 and have been for a while. Furthermore, I'm not aware of any changes I've made that could be responsible for this behavior so I can only assume its due to changes made with Apple's Siri servers. Do they have some financial motivation (through Yelp) to try to send us to businesses Siri "finds"?
So, to sum up: could the problem mentioned by the OP be Siri-centric and not iOS-version-centric? (Just a timing coincidence?)