2 diagnostics 1 week of studying in between(lower score by 100 on second one...)

-No studying took 1st diagnostic:

Mahattan GMAT(Free Practice):
650
43 Quant
36 Verbal

-Did 1 week of studying for the Quant section then took another diagnostic:

Princeton GMAT(Free practice):
550
40 Quant
26 Verbal

Obviously I need to study on my Verbal and I will but I am very surprised by the discrepancy... My Quant score did not improve despite having studied for about a week 2hrs/day, I expected my Quant to improve because I felt rusty on many of the formulas, permutations, etc that tricked me on the first exam. May the main reason be that both exams were from different companies? Manhattan vs Princeton? Has this happened to any of you? What score should I believe?

The experience was quite humbling. I was extremely disappointed with my 2nd score.. Any help on how to interpret the results and what to do moving forward would be appreciated, thanks in advance...