Clams have top and bottom shell.
Actually, it depends on the species. The pic above is the manila clam, and in that case, both shells are identical. For other clams, like oyster, horse clam, sand clam, and scallop, for example, they are not identical.
The manila clam buries itself in mud, and it does not make use of the camouflage on its shell. It is harvested by dugging up with fork, shovel, rake or a dredge.
When streamed or boiled, where the temperature is even on both shells, the meat will be on either side at 50/50 chance. The two adductor muscles (miniature scallops) usually are attached to the same side of the shell, holding the rest of the body (foot, mantle, stomach, etc) with it. When grilled and unflipped, the side closer to the heat source is cooked first. Since the adductor muscles only detached from the shell when cooked, they will remain attached to the top, which help to explains your observation. When broiled, where the heat source is on the top, the "meat" will most likely be on the bottom.