the same thing happened to me on Monday...I'm trying my best to find an excuse for why my score a 540 was 100 points lower than what i figured it would be, which i based on the 8 cat's (manhattan, powerprep and my first official gmat score (i got a 600).
Maybe your score and my score were so dramatically different from our expectations was that they threw in a new algorithm or a whole new test that started this monday? Idk really but this seems to odd to me... maybe its just a coincidence...
When you point to a post where others gmatprep was the same as there official score this was a long time ago - maybe they've change something?