400 Primary Chain Cover Wear Mystery

Good afternoon all!  So the primary chain is in within factory manual specs. Max is 20mm deflection/slack and mine is at about 11mm.  What could cause the chain to wear into the case and sight glass, yet chain guide is untouched (Looks like replaced before, no marks.)?  Remember, this is the same bike that the previous owners been known to install things backwards/upsidedown...   Yet, you can't install anything related wrong in this area. Everything looked right proper upon disassembly.  Thanks as always!
-Tom

My first guess would be that the po replaced a really sloppy chain with the one that you now have.