ToME: the Tales of Maj'Eyal

I am trying to play an Archmage now. The guy has a Command Staff ability : I activated it and choose
aspect -> mage, element : fire
Afterwards, the stats of this magestaff showed
When wielded/worn -> changes damage = +10% fire.

Then, later, I use again the Command Staff ability and select :
aspect -> star, element -> light.
The stats of the staff now show :
When wielded/worn -> changes damage = +10% light.

But when I use it to strike a foe (a feeble one BTW, it's just for the test), the combat log says :
Emilie hits Giant Rabbit for 11 fire damage.

I don't really understand how this Command Staff ability works...
What about the light damage bonus ?

Staffs used to work they way you're expecting, even as recently as a year ago. This made them crazy stupid good for melee magic classes that boosted specific damage types, and I think there may have been specific concerns with the Doombringer in development. A weapon's damage type is listed towards the top of its stat block. Most weapons deal pure physical damage, but some unique ones may deal other types. The +10% damage boost (here, on other items, or in skills) is a bonus to damage of that type. So if you're already dealing fire damage for some reason (say, you're casting Flame) you will deal 10% more. Bonuses form multiple sources stack.