Agreed. I think you are expecting the AVR to reduce the level of the center signal by 3db relative to the left and right signals because it's being sent to two speakers. It seems like the 3db increase is because you have two speakers replicating the center signal instead of 1. Level matching manually wont solve that problem. You should try running ARC with it set up as 4.1 in the speaker configuration and see if it is smart enough to drop the center signal by 3db relative to the left and right specific signals.