Reigning champions City host Chelsea on Sunday, looking to trim United's significant advantage which was stretched to 15 points on Saturday when they triumphed 2-0 at QPR.
Mancini's men face a huge task if they are to peg back their city rivals but the manager insists United have had the rub of the green on their way to the top.
"Until now, United have been really lucky with everything," he said.
"They have won a lot of games in the last minute - like against us when they really didn't deserve to win.
"So I don't think the gap between United and us is correct at this moment. They are doing better than us this season, for sure.
"But we have lost points in games we deserved to win, when we just couldn't score.
"For this reason, I say this is the difference. Last year we scored a lot of goals up to this point. We're maybe missing 10 goals and maybe seven or eight points."
City capitalised last season as United's title bid collapsed during the run-in and Mancini is hoping the pressure of their UEFA Champions League campaign will prompt a similar slip-up before the Manchester derby on April 8th.
He added: "If United go forward in the Champions League, the last two months will be hard for them.
"They could lose five points in two games before the derby."