Well, it's American food (more American than apple pie, as Jennifer 8 Lee points out) that started out as Chinese-American food and was further filtered through the taste buds of the American mainstream.

Just like "Mexican food" does not necessarily resemble what you'd actually find at a restaurant in Mexico. Certainly not the "Mexican food" they sell here in Boston; in Texas, at least, Tex-Mex food has a lot of the same ingredients as actual Mexican food. But, again, Tex-Mex is American food that started out as Mexican-American food.