But you can't tell the difference between 60-100 fps on a 60 hz monitor. But if it drops below 60 you'll see the difference. instantly.
To summarize wht zemerick said, people could often tell the difference between 60-100fps and 60fps because of microstutter, a hot topic in te graphics industry today. Fps is an average, and uusually the hhigher average will have less frequent or less drastic lows, but this isn always the case. Reviewers now test for microstutter and companies are wworking to provide consistency in frame rate.