Reasons for that are differences in the rest of test machine's spec, or more importantly, differences in settings (both in game and in control panel) and (maybe more importantly still) on what part of the game is tested. Obviously no game will deliver a constant framerate throughout - some areas are much more demanding than others. The best benchmarks will attempt to identify the most demanding areas of a game for worst-case-scenario numbers. Tom's does this but many sites just go with the first area in the game (or a built-in benchmark).