FPS => frames per second, depends on the pc's speed regarding the processor speed, ram size and graphics card performance.
If you see them with 100 fps, that means their computers have high performance. If they see you with 20 fps, that means your pc is not capable of playing the game smoothly. In order to see other players' fps, server should acquire a script which gets fps of a player like /getfps.
If the server doesn't contain a script that checks player fps', that means the other players who think you have 20 fps are whining about your lags and durability against the bullets.
Having 100 fps but showing other players as 20 fps, is actually impossible. Like David said, you should be mixing fps and ping.
