Unfortunately, this kind of fluctuation happens all the time, especially at the high end. It's very hard to get to the point where you can feel *certain* that you will score above 700.
If you post more about your experience, I may be able to offer better advice. What were your score breakdowns? How was your timing? Did you have to guess on many problems? Did both your quant & verbal move around, or did one sections stay fairly consistent?
My Quant score is very consistent with either 50 or 51 marks. However, my verbal score has varied from 34(on mgmat4) to 40(PowerPrep)
I scored 2 times 37 & 1 time 36.
On mgmat4; I hurried in the end and got 9 wrong of 11 questions; didn't finish last question and had to guess last 3 questions.
" Some are desperate for success, and therefore destined for it."