Hi JAPJEET06,

There are several possible answers to your question. The first is that there is always a margin of error on / range of possible outcomes for any test taker. That swing is usually +- 20 pts, but could be as much as +- 30 pts. So it's possible that your true range (at the moment when you took these tests) was 650 +- 30.

A more likely explanation is that your performance is inconsistent because either:
a) your content knowledge is not 100% solid, or
b) you're making mistakes in your techniques.

The first thing to do is to review whichever strategy guides you're using, and make sure that you know all of your rules backwards and forwards. Then, review both practice tests in a lot of depth. What kinds of mistakes were you making? Are there patterns in the types of errors you make? Your score won't improve until you fix these.

For more on tracking errors, see: https://www.manhattangmat.com/blog/index ... -studying/

No one particular test is going to be a direct indicator of actual GMAT performance. Only when you are consistently scoring within the same 20 pt range on at least 3 practice tests can you confidently expect to get that score on the real thing.