Yeah, because Phantom Axes aren't Assault. That's why people want Glaive/Hammer/Lance/Bow Grids. Since you get both Assault AND the base damage.
That said, for me, the Phantom Axe is about +-0 damage when compared to to a ~2200 Assault(++) slvl20 weapon, so I basically get 20 skill levels of Elaborate(++) and Exceed(++) for "free." Not sure how the Phantom Axe will compare to a FLB Hammer from the next Envy, though.