99% of all XP systems are completely dissociated from the game world. (The exception would be the RQ-style systems where using a skill or practicing a skill gives you the opportunity to improve it, but those are the rare fruit on the RPG tree.)
This means that XP systems do not generally exist to model any kind of actual learning process in the world. They exist to:
(1) Provide a convenient abstraction of learning, which is
(2) Tied to a reward mechanism which focuses the players on what they're supposed to be doing.
The default mode of D&D is exploration, treasure-hunting, and the slaying of monsters. Traditionally, therefore, most of the detail in the XP system focuses on exploration, treasure-hunting, and the slaying of monsters.
Where else are the ratlings going to get their dung bombs?
I've never seen anyone that plays this way.
You've never seen a scenario in which the necromancer is going to complete his evil ritual at the stroke of twelve midnight?
I'm calling shenanigans.
In general, analyzing this as some sort of x-for-y relationship that the characters can recognize in their own lives is like saying that parents in D&D-Land roll 3d6 to determine the ability scores of their children.
Sort of like how everyone ASSUMES that in an RPG, people use the bathroom. But even when you explore dungeons, you never find latrines or bedrooms. Even in published modules.
Rappan Athuk has a bathroom on Level 1.
And I do make a point of including outhouses, sewer accesses, expensive indoor plumbing, chamberpots, dung heaps, latrine trenches, and the like.