Brimstone from artifacts not getting fully applied

From one battle I got 72. I unequipped my +16 brimstone artifact and got 68. What am i missing?

guess if monsters are lower then your level you get significantly reduced resources.
otherwise need experimenting. i got it on arts/perk/rune. i suspect i dont get full value either. since arts come with +50% power boost which IF working would show that base amount of brim i get from killing higher level monsters on high level realm is about 50 brim. before modifiers. which is unlinkely