I am not sure if this is valid, but I guess so.

When you turn on Vsync, which limits your FPS to 60 (at least for me), then minimize Minecraft and reopen it, you will see that your FPS aren't restricted to 60 at the beginning. This seems to happen only it Minecraft is minimized. Having Minecraft and another program open (next to each other), but the other program is focused doesn't cause this.

Note: To see the FPS graph press Alt+F3