Bang for the buck = efficiency, so you just answered the question "Why is it more efficient?" with, "Because it is more efficient."
The reason, more or less, is that AMD GPUs tend to have way more cores than Nvidia GPUs, and their cores can, in hardware, do the integer operation necessary for the hashing algorithm. Nvidia can only emulate the integer operation, so it takes three instructions on Nvidia vs one on AMD.
The reason, more or less, is that AMD GPUs tend to have way more cores than Nvidia GPUs, and their cores can, in hardware, do the integer operation necessary for the hashing algorithm. Nvidia can only emulate the integer operation, so it takes three instructions on Nvidia vs one on AMD.