Hi asocialnot,
First off, a 620 is a decent score (it's a bit below the 80th percentile), so it might be enough to get you into Business School. If you're planning to apply to any highly competitive programs, then it's understandable that you would want to retest.
You've defined a big 'issue' with how you studied - you did NOT make your CATs 'match' Test Day. The accuracy of a CAT result depends on a number of factors, including how 'realistic' YOU make the experience. You should take the ENTIRE CAT (including the Essay and IR sections), away from your home, under as-close-to-'test-like' conditions as possible, at the same time of day as your Official GMAT, etc.
There could be other aspects to your studies that you need to adjust (or completely change), so I'd like to know more about how you approached the process:
1) What is your score goal?
2) How long did you study?
3) What resources did you use during your studies?
4) When are you planning to apply to Business School?
8 weeks of additional study should give you a great opportunity to improve, but you might need to invest in some new resources to help you reach your goals.
GMAT assassins aren't born, they're made,
Rich