The difference between Heaven and Runescape is how much of the resources are used. A synthetic benchmark stresses the specific hardware (in this case, your gpu) to more substanially using 100% the entire time. Runescape is a real world game, and although it does use the gpu, it will at no point stress it like the benchmark.
You should not be concerned about it being clocked higher in some games. It will not be doing the same damage like a synthetic benchmark.