It has to do with the fact that true assassins need to have an in-depth knowledge of biological science, what arteries to sever, what nerves to cut, where exactly the bones on the temple meet to shatter with a well-placed strike PLUS most assassins learned this stuff for use in the cities, how shadows fall in the cities, how to walk unheard on specific roads when slinking up for your assassinate, ect.

....or it's a game balance thing like the guy below said.