Just look at Inter when they can't count on one or two important players. Maicon in the previous round, Muntari and Samuel in this round.
We played 19 matches, always missing at least 3-4 very important players. We were missing our engine and most creative midfielder, our best "defender" and organizer of the defense, Buffon, we were missing Zanetti, Zebina. On every match we have one aditional injury/suspension that creates many squad problems and we're always missing at least 4 bench players which prevented us from rotating players.
Salihamidzic, Tiago, Trezeguet, Iaquinta. Someone may say that we have equally good players, or even better than them, in the starting XI, but there is a big difference between having them available and not having them at all.
The main reason, by far, for not leading the standings by 3-5 pts are these anoying injuries.