Framerate is an average amount of frames produced per second. This doesn't say anything about the pacing. With consistent pacing, a game running at 60fps should produce a new frame every 16.7 milliseconds. However, for instance, if half of those frames took 33.4 millseconds to produce, and half took 8.4 milliseconds, you are still getting 60fps, but it will feel choppy as the frames aren't being produced consistently.