Step 2: probably only deemed it necessary for those areas to be somewhat protected from moisture and rock chips ( just a guess?). Or, paint was needed where dealer undercoating was applied, so as, to adhere to? If a penny could be saved by not painting an area, GM, and all the manufacturers, would save where they could. No 2 cars are identical on the bottom. I have seen generous application of any and all the colors and types of prep coats put on the underside. I have a car that a good bit of the area from the dual exhaust front hangers on back is painted the body color, yellow. It is like someone was cleaning their gun out and sprayed the entire area. The noted paint recommendations are a guideline, and should be taken as such. The human factor as such, maybe the guy was in a bad mood or hung over or going on break or any number of scenarios? He just did not do his painting with any care or hurried through each car. Maybe the painter liberally applied paint on several cars until a supervisor who was cost conscious put a stop to the perceived waste?
As a side, the rockers are painted with the gun kept vertices and perpendicular to the rocker. The underside of the rocker does not get painted, except for overspray.
