i've always thought that seer's knowledge of their aspect is very. results oriented? Kankri might know a lot about the social connections between people, but that doesn't mean he's good at it. Rose's powers show her things and events that are important, but they don't show her why. in contrast, maybe Mages understand their aspect on a practical level? sollux knows the ins and outs of Doom, and he's able to exploit that knowledge almost like video game cheats (biggest example I can think of is that he definitely has two dreamselves because he set that up when he was programming SGRUB)