Not defending Stern and this can actually be a criticism, but when talking about old EM's or a Flash etc and probably most B/W games I do not recall such games having many if any Airballs. Now that probably is because they were well designed and in the case of EM's things are moving much slower. So if a ball is bouncing around the playfield a lot, multiballs a plenty all contribute to dimpling.

So this could be one explanation or contributing factor on top of softer pf's. What caused divots around ramp ball drop areas on B/W games. I'm guessing thousands of balls dropping in the one spot. My thoughts anyway