I took the GMATPrep test 1 twice and got VERY different scores in quant, infact a lower score the second time when I think I performed better.
attempt #1: 12 mistakes, raw score 48
wrong answers: 3,4,5,7,13,16,18,25,26,33,36,37
one streak of 3 consecutive mistakes (and that too pretty much in the beginning), 7 mistakes in 1st half, 5 in 2nd . 4 mistakes in first 10.
attempt #2: 12 mistakes, raw score 43
wrong answers: 1,6,9,11,13,15,16,17,24,28,36,37
only one streak of 3 consecutive mistakes, 8 mistakes in 1st half, 4 in second. 3 mistakes in 1st 10.
The ONLY possible reason I can think of is that experimental questions screwed up the scores - getting no exp. qs right on 1st attempt and getting all of them right in the 2nd one.
But it would still be very unfair as it could easily swing the final score by upto 50 points.