Why do hamburgers have one streaked/furrowed and one smooth side? Which side should be cooked first in a pan?  I'm talking about their shape, not their color or anything similar. If you look at a raw patty from its side it looks like the drawing below.  --------------------------- |  _   _   _   _   _   _  | |_| |_| |_| |_| |_| |_| |_|                 You can see the ridges I'm talking about on this photo. 
The main reason that your burger has these ridges on only one side is simply manufacturing. The ridges are put into the patties to prevent it from the middle coming up on you like bologna does sometimes when you fry it, and to also promote even heating and cooking. The only real reason most companies have for putting it only on one side, is that one flat side is the side they use to move the unfrozen patty around. There is a spatula type device that moves them around during production. If they slide something under the side with ridges before the patty is frozen it would mess it up before it was frozen. So you only see the pattern on one side on most of your mass produced frozen patties. Secondary reason is smaller but, some companies place pieces of wax paper between patties for easier separation after freezing, and the smooth side is the side that the patty rests on during or after freezing because it sticks to the wax paper more easily. 