Armor Set Stats

So I was wondering how the stats from armor set are calculated.
(If this treat already existe, plz tell me).

Adventurer's set:
My helmet : +5% xp bonus; +3% loot bonus
My Chestplate: +3% loot bonus
Leggins: nothing:
Boots: +5% xp bonus

set bonus: +20% xp bonus, +15% loot bonus

and necklace: +4 % loot bonus
mage's weapon: +5% xp bonus

So, Im supposed to have: +38%xp bonus and +25% loot bonus

But: in my caracter information : "+15% xp bonus" and "+10% loot bonus".

Can you tell me if I kill a mob if there is a +15% xp bonus or +38% xp bonus?

I tried to kill the the same mobs to see it but each time the xp is different.

Thank.