In order to evaluate GPU performance, you try to eliminate CPU limitations as a factor. That means running at high resolutions and with the fastest CPU you can possibly get. This will (hopefully) cause the game to be almost or entirely GPU limited, and therefore give you the maximum possible amount of information about the relative performance of various GPUs.Explain then.
Conversely, when evaluating CPU performance, you will chose the fastest possible GPU setup you can get and use a modest rendering resolution. This allows you to accurately compare CPU performance in a CPU-limited scenario.