I had thought something had been off with my damage when I took it, but I wasn't sure.
It seems when I use it on a great-axe, the damage increases beyond the range of a 3d4+strength. But when applied to a double-axe, the damage is less. I performed a number of combat tests for a while to see if I could ever roll a physical damage number as high as the greataxe, and I could not.
I previously posted a screenshot of the misc average damage readout, but that wasn't an accurate resources of information. Regardless, both weapons are a 3d4, but the greataxe is clearly benefitting from improved wepaon hex upon testing, whereas the double-axe is not. When unhexed, the highest I can achieve on physical damage with both is an 18. But with improved weapon hex, this range increases for the greataxe.