I'm not sure if it's something that is intentional, or if it was missed. I couldn't find anything about it in forums (though admittedly I'm still learning how to navigate it).
I just used a voter book for that wonderful bonus exp for an hour of superheating magic exp. However I noticed the bonus exp applies to the smithing portion, but the magic exp portion remains the same.
I definitely expected them both to be raised. I'm good with it if it's intentional because that just means more bars to smelt! But if it was simply overlooked, I'm sure lots of people would appreciate them both to be increased!