I think GMAT is not a test to calibrate person ability nor it can, however, I think it is a test of self-exploration where one comes face to face with his own weakness in general. I will later explore and shed some light on this point in my debrief.
One of the weakness that I realized is, that failure beget fear which is why a lot of people underperform in the test. Eg if some quant or verbal question is done wrong when revisiting that question over conscience to get it right or overly concentrating on it make it even difficult to answer it correctly or takes unusually long time, even in the second attempt, although you know the concept this time around.
Taking this ideology and looking at the bigger picture. If a person underperforms in mock or 1st actual attempt the fear builds which not only makes test taker overly concentrate on the test but all together makes it even more difficult to score well than it actually is.
Solution to such a dilemma is to let it float.
Regards,
Ali