Modern laptop have varying degrees of power saving modes. Running things that require more from the laptop will bring you out of the current mode and up to a better performing mode. For example if you aren't really doing anything, say reading a webpage, one core would be active and probably running at 800MHz. change page and the cpu would jump up to full speed on one core, launch an app and you might be at 2 cores at full speed. Run a game and you have all your cores running at full speed plus the GPU is now active too. This is usually your most power consuming state hence you get the least time while on battery.