I don't know if the XP has some form of scaling dependant on the Level on the hero but I've manage to find a bug where you can level a level 1 hero to level 19 by completing the ITER. See attached photo's of the hero before and after and check the turn.

Normally completing this building with a hero around lvl 6-7 will give it 2 levels. So this is the reason why I think there is some kind of XP scaling issue.

Mmh I think there's indeed some sort of scaling but I don't think such jumps in XP should happen.

I'm going to relay this thread to the rest of the team since I think a rebalance of Hero XP is planned at some point :)

That'd be the perfect time to fix issues like this one!

Cheers :)