They paint the body on the frame to ensure that everything is aligned, and that the passes the painter makes with the gun go across all body parts the same number of times and in the same direction. This is particularly important with candies, metallics and pearls.

Whether it shows on TV or not, most shops will have the bottom painted or undercoated by then. If they don't, they will pull the body back off.

Or - they might just have stock footage showing the car rolling into the paint shop and show it out of sequence. Happens all the time.