I've also found that Zune keeps track of plays for each user. If I log in with my Zune Pass account, I see plays for it, and I see a different number of plays under my personal account (used to keep a list separate from the wife). When I download music with the Zune pass, I have to log in later with my personal account and re-sync or I'll have playlists and plays from the Zune Pass account.
You may not use Zune Pass, but Zune really needs to allow the Zune Pass to be tied to more than one account (up to three computers and Zunes are allowed).