Another reason, it depends on the kind of scope, the size of the front collective and the clearance of the scope to any of the weapons funtions exp. (bolt rilfe). Here's a vid from Midway USA on sellection of a a scope height. (Note: it's a little different on a semi-auto, but the same rules apply with them as well).

http://www.youtube.com/watch?v=gpLi064e_JI