Medium and hard throw in new kinds of operations, but how hard a problem is for me at least seems to mostly scale with the number of digits I have to juggle, not the operation "complexity".
E.g. if I'm at 14, an "easy" multiply "by itself" forces me to multiply long-form (I don't even have 4 x 14 in my mental LUT... so 140 + "uh... 56 plus..."). On the other hand, again starting at 14, a "hard" multiply by 4/7 instead is trivially 8 (mentally rewriting to (14/7)*4)
E.g. if I'm at 14, an "easy" multiply "by itself" forces me to multiply long-form (I don't even have 4 x 14 in my mental LUT... so 140 + "uh... 56 plus..."). On the other hand, again starting at 14, a "hard" multiply by 4/7 instead is trivially 8 (mentally rewriting to (14/7)*4)