Basically, a pc game has to go through the operating system and is micromanaged by windows through your hardware, where multiple programs are running.
However, at any INSTANT in time, a processor core is only processing one set of binary instructions at a time (hyperthreading, packing, everything else aside), so if your operating system allocates a few instants in time to another process, you can actually see a jump in a moving picture on your screen, however small, and your framerate detector not pick it up.
A console doesn't have an operating system, it is basically a machine configured with an ISA(instruction set architecture) to do a few distinct different things, and the games are programmed to that ISA instead of being programmed to an open-source type environment that PC games are written to.
The point of all that is to let you understand that a console game interfaces directly with the hardware of the console, where a pc game (even if written completely in assembly) still has to be micromanaged by your OS, where regardless of how awesome your machine is, can make it jump every once in a while.