I'd say it's because certain individuals are improving in different ways. Also new faces in Clichy, Nasri and Aguero have helped. We are playing more as a team as well.
Kolarov has had to up his game attacking wise meaning he has played better, scored and assisted some.
Dzeko has had to prove he can score goals in the best league. He has done that and expect him to have exceeded many peoples opinions already.
Balotelli has matured and has turned from liability into a top class striker, just like Kun and Edin.
Milner has had to prove himself with Nasri coming in but he has definitely lifted his game. As has Barry IMO.
Playing with 2 top class strikers a game also helps goal wise.