Hi ksoydanyavas,
Your practice CAT scores are not as far apart as they might appear. 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 practice CATs function, these scores all point to a general ability in the mid-600s.
This ultimately means that you continue to deal with your CATs in the same general ways that you did before. You do certain things correctly (as you did before) and you make the same general mistakes (as you did before). The 'issue' here can be any of the following:
1) You learned/practiced the proper tactics, but when the clock starts ticking, you don't use them.
2) You have developed a series of 'bad habits' that lead to silly/little mistakes and you haven't done enough to remove those bad habits from your work.
3) You haven't actually been practicing the proper tactics - instead you've just been doing practice problems in the same ways that you've always done them.
It's also worth noting that at this scoring level, the GMAT becomes really 'sensitive' to little mistakes. If you make too many of them, then a 700+ score is simply not possible. To that end, you should do a full review of your CATs and count up the number of questions that you SHOULD have gotten correct, but didn't because of a little mistake. THOSE questions represent a big 'chunk' of your missing points. To improve, you cannot simply assume that you 'won't make that mistake again' - you have to 'drill' and train to remove even the smallest possibility that those same mistakes could happen again.
GMAT assassins aren't born, they're made,
Rich