Hullukko, it is well documented that different weapon types in PL had different skill damage modifiers. Ever see birds still laying waste to mobs with talon/wing combo by spamming skills? That is because they didn't lose much skill damage. Auto attack damage was much lower, and thats why most of those birds switched to a bow at bosses.

In SL, of course the plat rifles do more skill damage. They are premium weapons, and do higher damage most of the time than dropped rifles. Compare skill damage with dropped weapons of various types and same level, and you will find that skill damage is very close if not the same.