Hi Anand,
The scoring algorithm on the Official GMAT is far more complicated than most people realize. Since that algorithm is proprietary, no GMAT company has an exact match for it, thus CAT scores can vary a bit based on the 'biases' involved in their respective designs. The most realistic CATs available are the 6 from GMAC, but retaking a CAT that you've already taken is NOT a realistic way to assess your skills - since you will likely see some 'repeat' questions that you have already faced. Unfortunately, seeing even a couple of repeats can 'throw off' the Scoring Algorithm and impact your pacing, energy levels, fatigue, etc. (meaning that they would likely all appear to be better than they actually are). Thankfully, the CATs from Kaplan,
MGMAT and Veritas are all 'close enough' to the real thing that they will provide you with a relatively realistic score assessment (assuming that you correctly take the CAT in a realistic fashion).
GMAC has publicly stated that the Official Score that you earn on Test Day is within +/- 30 points of actual ability. Assuming a similar 'swing' in how your CATs function - and we ignore the two "outliers" (the 680 and the 500), then your 3 recent CAT score results - along with your prior Scores - show that you essentially performed the same each time (about 610 +/- a few points). You handle certain aspects of the GMAT consistently well, but you also make certain consistent mistakes. It's possible that you've developed some 'bad habits' during your studies that are keeping you from scoring higher.
With a March 10th Test Date, you have about 4 weeks of study time remaining, so you could potentially improve enough to hit 650+. However, if you plan on studying until you hit 700+, then you will likely need at least another 2 months of consistent, guided study to get to the point that you can consistently score at that higher level.
Enhanced Score Reports are only available for the Official GMAT - and you can order one even for a 'cancelled' Score. Is the prior 610 you mentioned from a practice CAT or from the Official GMAT?
GMAT assassins aren't born, they're made,
Rich