if you look at a les paul, the neck comes out of the body at a backward angle, so the strings are further away from the body at the bridge than at the neck, so the bridge mounting ring is taller than the neck pickup ring. they're also curved to match the top carve. then there are flat bottomed ones for non carved top guitars. of course, not all guitars have an angled back neck, on my eggle berlins both mounting rings are the same.
hope i've explained the right thing...