When I read about the 4 journey roles, I assumed the procedure was meant to split up the duties among the players. Since the guide makes all the pathfinder rolls, that hero is already sort of determining if the whole company is going to get a longer or shorter trip based on those rolls, e.g. a bunch of bad rolls and the company only moves 1 or 2 hexes between events compared to 3-4 for better rolls. So the guide's rolls also play a part in the fatigue DC modifiers as part of their duty simply by making all the pathfinder checks. For each event / pathfinder roll, one of the other 3 roles will make ability checks based on the event resolution procedure, almost all of which can affect the DC modifiers directly depending on success/failure, etc.
If we let the guide be a target of an event for a given leg of the journey, then the guide "gets" to roll all the pathfinder and event rolls while the other players are doing nothing for that leg.
From a role-playing standpoint, I just think of it like this: the company ends the day's walking so the guide sits at the campfire looking at the map, the lookouts are climbing trees and cliff-faces actively watching for danger that might invade the camp area, the hunters are out trying to procure food, while the scouts are out trying to find the best path for tomorrow's journey. It makes perfect sense to me why the guide wouldn't be near as likely to trigger an event under those circumstances.