This is even more impressive having in mind that last season we also had CR7 and Tevez and most often played with 2 or even 3 forwards.
One reason I can think of is that I read some statistics that CR7 alone took more than half of United shots-at-goal last season - he was quite selfish.
Rooney, Berbatov, Valencia, even Nani and Giggs are much less selfish and therefore the whole team scores more.
Rooney, Valencia, Berbatov all have great goal scoring/game ratios (Rooney is 20 goals in 23 starts, Berbatov is 8 goals in 14 starts, Valencia is 5 goals in 18 starts). When the players are not selfish everyone scores more.
If we had the same great defense as last season - we would be running with the title. Last season we had 11 clean sheets in a row about this time of the year. We obviously have the best offense in the PL this season - lets hope the defense recovers from all the injuries and begins playing as well as last season.