I don't know how they finish them or if the lines are a product of the mold.
Actually I'm going to say it occurs afterwards as I have some white blanks and they have a mirror finish.

Probably reasons for that...
Slightly better grip when stacked.
Texture allows better adhesion of the dyesub film
Design wears better

I've seen some with pretty coarse parallel straight lines across them. I think it makes them look 'cheap'