Just playing devil's advocate here...

The real GMAT has a margin of error of 30 points, so let's say that a practice exam - which is just a guess as to how the real GMAT works - has a margin of error of 50 points.

Let's say that you underachieved by 50 points last week, and overachieved by 50 points this week. So, both of those scores are within the margin of error. Your true score is probably somewhere in the middle. Whenever you look at a score from a practice test, imagine it as a 100 point range rather than a true score.

That said, I *have* seen people go up 100 points in a week so it's certainly possible - I just don't want you getting overconfident until you've gotten mid-600 results on a couple more practice tests.

Depends. Was 540 the first time you'd ever taken any GMAT practice exam? My first practice test, I got a 480 -- I just was unfamiliar with the GMAT format. One week later, I got a 650.

However, you'd need more scores to tell if the 640 is a true outlier.