As I understand, GMATFocus gives an 80% confidence interval only - there aren't enough questions for it to do better. That means the test was 80% sure you were in the 48-51 range, but that there's a 20% chance you're outside that range- maybe you had more lucky guesses than normal, say. If, however, you take two or three GMATFocus, or GMATPrep, tests and consistently score in that range, you can be confident that your true ability is at that level. That said, how you perform on the GMAT depends on a few factors, as aim-wsc suggests. Your ability matters a lot, of course, but other factors can play a role- nerves, tiredness, etc.