933 has a solid roller cam. Because the lifters are solid, they don't flex much as the engine wears, so they are adjusted periodically to insure proper valve closing. Hydraulic lifters adjust as the engine wears because the lifter telescopes based on
oil pressure in the lifter. I'm sure there is more involved there (I'll leave that to the experts), but the main point and bottom line is solid = manual valve adjustment and hydraulic = self adjusting valves.