As I understand it, the only reason the official score changes from the unofficial score is if the actual scale of the exam moves. For example, 750 used to be 99th percentile, but now it's only 98th percentile. If you got 750 on your unofficial result, the percentile would be adjusted to the current score at the time your official score is processed.
I haven't heard of actual changes in score between official and unofficial. I actually always believed that it was a way for them to do a security check before issuing an official score - to make sure everything was OK at the test center, your biometrics matched, etc. I'm just speculating though.
I think he is talking about the practice tests vs the real test.