With step: 60 in createGame, delta is always 1/60, a jump is the same height everywhere, and x += 2 is two pixels a frame.
The bell hangs half a pixel below the top of the jump, so every jump rings it. Change step: 60 to step: 'display' in the code and run it on a fast monitor: the jump comes out a pixel or two lower and misses, and the updates per second follow the screen.