Saved into Contacts? Actually, no idea.
It's possible that Gmail itself (the server, not the app) is doing it. I believe they have a setting somewhere in web mail.
A quick way to check is to remove this contact from your Contacts app, and then check when exactly it "comes back": when one of these messages is received, or later, when you view it in Aqua (my guess - not at this point), or after you've saved the event into your calendar.
And that's one more app that could be doing the saving -- the Calendar app (actually it's more broad than that -- how Android's built-in calendar support handles events saved under a Gmail account).
