"This is going to be interesting."
I think the better question is why are the skills called Man-at-Arms, Expert Marksman, and Scoundrel when the game files refer to those skills as Warrior's Lore, Ranger Lore, and Rogue Lore respectively?
This part, at least, is an artifact of the skill system's development. They were shown as "Way of the Warrior/Ranger/Rogue" until a few beta versions before release, and at that point they did not govern the number of skills known in each category; instead they provided various benefits that were subsequently made into traits under the current model.
The varying names also fit with the rest of that category: there you also have relatively convoluted but differentiated terms like aerotheurge and hydrosophist instead of "Air Magic" and "Water Magic".
