i have noticed the same thing on mine, but haven't had time to play with the settings in the iDrive to see if adjusting anything helps. i remember there being the option of adjusting where the air is coming from, but i think that's only floor, dash, and windshield (no differentiation between middle and side vents in dash, if I recall...).

And I do have the ventilated seats, but don't think that makes a difference.