Does the difficulty levels of NMAT by GMAC vary across different windows ?

NMAT is not much difficult as compared to exams like CAT. It depends one or so set may come a bit less difficult but its not a definite thing. There is nothing as such fixed difficult paper.
If you have your basics right with good calculation techniques then it is going to be cakewalk for you provided you are well versed with the formulas. Lot of questions are formula based.

Hope this helps.
Good Luck.