Another thing when a Games gets released on both PC & consoles there is a difference in how bullets travel. By this I mean on the console side the same weapon will have a wider spray pattern if I can use that expression & the hit box on the characters will be a bit larger thus giving the people with gamepads a similar level of accuracy as mouse users.
I'm not trying to say someone with a game pad can't do well or beat a guy with a mouse. I used to play the Original Unreal Tournament with the MS Dual Strike gamepad that was designed for FPS and I used to do very well. Even made the top 100 for a specific week.
PS. Cabezone made a very good point the user can make a big difference.