Getting a score a bit lower than wajt you started with should not be the sole reason for deferring your Test. You should delve a little bit deeper and try to find out what went wrong. I have a similar experience - when I started out with my GMAT preparation, I took a GMATprep CAT just to get the feel and I scored 690; hoever, after 2 months of decent preparation, I took the "same" CAT (with 10% common questions) and I scored 700. So my improvement was not much.

What I think you should do is to go back to the last CAT and categorically analyze your mistakes. Try to separate them into different classes (for e.g. Verbal - CR/RC/SC and then CR-assumption/strengthen/weaken, method of reasoning etc) and this will help you find out where your problem area is.

Since your schedule for the Test is in two weeks, I will strongly suggest that you do this analysis before deciding one way or the other. Getting a score of 530 with expectations of 640 is a big drop, so check your last performance. Also, try to take one more test and see if you repeat the same kind of mistakes. Your score in this one test may have been just random, may be your score will be much closer to your expectations in the next one you take.