Hi tacet22,

The short/simple answer is that you made a bunch of silly mistakes. The GMAT is remarkably good at giving you the score that you EARN. If you don't do enough work then you won't get the points. The likely category is DS, since nobody realizes when they make a mistake on a DS question.

The more complex answer involves HOW you studied, how you took your CATs and what you did in the week (and the last few days) before your Official GMAT. To that end, if you can answer a few questions, then it should be easier to figure out the factors that cost you those points:

1) Did you take the ENTIRE CAT every time (or did you skip sections - notably the Essay and IR)?
2) Did you do anything unrealistic during your CATs (pause the Test, listen to music, take the CATs at home, etc.)?
3) What time of day did you take your CATs and your Official GMAT?
4) How many CATs did you take in the last week before your GMAT?
5) What did you do during the last 48 hours before your GMAT?
6) How did you sleep the night before your Test?
7) Was there anything surprising or distracting about Test Day?

The good news is that Business Schools don't care if you take the GMAT more than once. Also, since the GMAT is a predictable, standardized exam, you CAN train to crush it. Before you retake the GMAT though, we need to define (and fix) whatever went wrong.

GMAT assassins aren't born, they're made,
Rich