I was noticing a while back in another release .... mw2 I think, my son and I were playing in the same lobby. Every match I'd have more xp than him even though he usually di better on the scoreboard.
I told him I just use everything and do everything I can Throw nades equipement etc, anything I could think of to do.
Maybe the scoring in this contains more stuff that was just included in your bonus, but never toward the scoreboard.
I havent even really noticed how any of the xp goes ..... I've just been playing.