I thought it was also that
I thought it was also that the ND drive train was much more efficient in getting it's 15Xhp to the ground. I vaguely recall seeing dyno plots someone did of an NC vs ND miata and even though the ND has less rated HP the hp to the ground was almost identical if not in favor of the ND. Doesn't the fiata use an NC trans as well so that + weight could account for the differences.