I think that would point more to the valve body.
Talking aloud here.....
When you put the car in D, 3, 2, or 1 the foward band is engaged by the servo. When you drop the car into 2 or 1 the manual 2-1 band is also engaged along with the forward band.
The gear selection for 1 or 2 still allows the trans to shift into 2nd when it reaches certain parameters, it will hold 1st longer though. This IIRC is based on pcm programming and not the valve body.
Choosing 3rd on the gear selector keeps fluid from reaching the 4th feed for it's clutch piston.
