I don't see what the issue is, absorb is stronger since it also heals you - so it should be lower, what am I missing?
Basically if you make a weapon with Absorb anything/Fire it'll do 36 fire damage. If you make a Fire/Shock weapon it'll do 45 fire and shock damage. I'm assuming you could do it with frost too but there's never a good reason to use frost (especially since it puts out burning enemies).