To be honest, I'm not really sure how you could explain away experience boosts… Sure, damage boosts == endowed powers... pneuma boosts == the gods (or just your god) favors you... quest boosts == dude the king rock man! he like totally rocks!!
One way for the boosts in general to exist outside of say, your spells or the global boosts, is have anchors to what triggers them. If/when deities are added, it could be a parameter of said deity that has a chance and boost factor that can give individual/group boosts, much like aligned churches have. Could require things like high favor with your god(dess) to gain such boosts.
QP boosts are rather simple for backstory... you please the particular questmaster, so you could get boosts from that QM for a period of time, from generocity and appreciation of service.
Damage boosts, I'd say make THAT what reckonings give, not experience boosts. I mean, c'mon, the reckoning makes you want to KILL!KILL!KILL! Surely all that rage and aggression causes more damage... In the long run, double damage oft results in double experience given that you kill mob twice as fast.
The only one left is the XP boost. Mayhaps have a state of enlightenment that opens your mind (be it through drugs or an actual journey of the mind) and alters it in such a manner that you gain experience at a faster rate.
Those are my two cents.