The blend door only controls how hot or cold the air is, i.e. blending or directing air flow from the particular apparatus that dictates the temp of the air you want. As long as the air coming out of the center vents is HOT or COLD (w/AC), then that door is working as normal. What is the temp out of the passenger side-side vent? Is it the same or different +/-?
I guess the "circulation" door that direct the air to the particular part of the car - torso, torso/floor, floor, floor/defrost, defrost is the door that might be causing the issue.
The side (outermost) vents never really get that much air velocity and due to the distance traveled, the air coming out of them can loose a lot of thermal energy by the time the air makes it to the actual vent.
I do not know how long the ducting is or how many bends it makes or what the thermal properties are of the duct material itself is, but again, the side, door vents are not the main distributors of air. Now, if you cut air flow to the center vents (not sure if this is actually possible), then the side vents begin to push more air, but again, not at the levels that the center vents do.
I guess, if you want to, you can close the passenger side vent to "force" more air out of the other three torso vents.
My thinking, the shorter the run, the hotter the air...the longer the run, the more time the air has to "cool" off.
Have you verified the temp using a temp gun; a few bucks at your local HF.