I am not exactly sure how Gmail’s spam filters work, and likely won’t figure it out anytime soon – I flunked out of my Astrophysics PhD program, don’t you know. But I do know that spam seems to be a very subjective thing at the Google service. Some people get it, and some don’t.
I still haven’t received any through the service, and check my Entourage folders diligently. I use the POP service at Gmail, so I leave no messages on the service except those in the spam folder (which I empty roughly once every 30 days). But the real kicker is this – I use my Gmail account for all web forms, etc., considering the account disposable if need be. I get all the spam from my primary domain, which I guard with my life.
Gmail’s spam filter may learn from the spam you report, but does it get dumber from the email you keep?