Practice Tests - Discrepancy in Scores

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Hi..

My GMAT has been scheduled in the 1st week of September 2008. I have been taking some Practice Tests, the scores of which are as follows:-

GMAT Prep - 650
Kaplan (after 10 days) - 580
Online Manhattan Test (5 days after Kaplan)- 660
Princeton (2 days after MGMAT) - 590

I have gone through various posts that explain that Kaplan Tests are low scoring and that one can add between 80-100 marks to get an approximate idea of how one is likely to score. (plz correct me if i'm wrong here!)

I was however very puzzled by my Princeton GMAT test score. (I was positive at the end of the test that I was nearing the 700 mark.. or that I would've got atleast a 680!) I got fewer questions wrong than in my GMAT Prep, Kaplan or Manhattan Tests and still managed to get a score of 590!

Can someone plz explain if u've had a similar experience? or throw some light on how best I can interpret my Princeton GMAT score?

Thanks a lot!