I have never heard of the score changing between the unofficial and official report (althogh i have heard of GMAC canceling people's score a month or two after they take the test--yes, this made those people furious!)
But maybe people are just worried or hoping that the score will change between the unofficial and official score report.
When I got my GMAT score on test day, i was really happy with it. So I was a bit afraid (in the back of my head) that the official report would arrive with a lower score along with a letter stating that they made a mistake or something. For me, it was just paranoia!

I think a lot of people with good scores hold their breath until they actually see the official score. While people who wish they'd received a higher score cross their fingers in hopes of some rare chance that the official score is higher.
Just my two cents, I could be completely off base with what you're asking.