You got lucky. There've been scattered reports of people with phones like yours; the theories I've seen are that someone's entering random ECID's into the system somehow, that you may have gotten a phone that was returned and resold as new, or that Apple employees may be doing stuff like this just for the hell of it. Regardless, you're part of a very tiny minority, congrats!
The same trick didn't work on my husband's phone on the afternoon of the 4.1 release. I tried it when reports started coming out about the proximity sensor getting worse with the upgrade; thought I'd save his blobs just in case he ever wanted to downgrade. No dice, it was already too late by then.