The Celtics are a bad shooting team, by NBA standards, but they have a decent offense because they put up an ton of shots due to the pace they play at (some of this is also due to them being good defensive team, and getting turnovers).
The Offensive Rating that people use to measure "efficiency" takes that into account since it looks at how many points they score per 100 possessions, which is roughly how many you would expect in an NBA game.
Like Doc said, it is a "make or miss" league. At the end of the day guys have to put the ball in the basket to win games. Recently the Cs have done a lot more missing, hence the losing. Last night, they shot better and voila - they won.