I think the difference is that the veneer used on the older playfields is thicker and harder. The Stern pfs are much softer...
CPR even says they use a 25% thicker veneer than Stern...
Added a few pics of my WCS'94 this game is air ball city (spent most of its life on route) and the damn thing hardly looks like it ever took a dimple. There is no way in hell my GB pf will ever look this smooth after dimpling.