I noticed you just aquired new weapon which makes my wild guess bit harder... Average ilevel doesnt matter that much if you're wearing lower ilevel weapon due to sheer amount of spellpower you get from weapon. Also remember that in raid enviroment you have 20% increased spellpower from buffs (inner fire and mage buff, you are using inner fire right?).
Currently you have with new weapon 22.8k sp *1.2 = 27.36K
Other guy 28.2k sp * 1.2 = 33.8K.
Smite coefficient is 85.6% so ---> (33.8K-27.36)*0.856 = 5.51K. You have roughly 3.5K difference in intellect. This should be pretty close the difference you and him have now. What weapon were you using before? Or do new log with new weapon? He gets some more stats from buffs due to higher intelligence but that is worth only 200 damage per smite.
The numbers are off a lot since there's ~17k difference on average smite damage. For comparasion I have 24.2k spellpower and my smite hits for 47k average in raid enviroment without damage buffs (checked from log).
So overall seems really bizarre and I can't think what's causing it... Best way would be to do new log with your new weapon and compare the numbers then. Sorry I couldn't really help. Maybe someone notices error in my post and gets us on right track.