My old AFM and routed STTNG has no visible dimpling and those are fast games. The AFM LE here has extremely noticable dimpling:
and a new Star Wars LE which is on location with relatively low play has a very dimpled playfield. So I am very confident it is really caused by a difference in materials.
Those german overlays work to protect the playfield, but the location that uses them has many stuck balls because of those overlays.
My Doctor Who is just perfect, why don't they make diamond plate playfields anymore?