I concur with you guys that wearing off a plastic surface by just typing on it is very unfeasible, to say the least; while, in the other hand, the theory of a molding defect in the second shot of a non filled cavity in the mold is fairly common, even in single shot molding. The last part of it, that the "set" is just a collection out of crap bags is the last event to conclude that these key caps are not worn off ones, but just defective parts out of the molding process.