^ mattc spaketh some sense. ( )
Also, rider's local knowledge comes into play even if they aren't local. Some orgs dont mind busy main roads. I hate them and if I can find a quiet alternative I'll take it, even if it means more hills. Organisers are (I think) obliged to take the shortest route between controls...but this doesn't mean it is the nicest route depending on the POV of the riders. Rather than bitch about it, I'll just take an alternative.
No, the org's suggested route does not need to be the shortest distance between controls.
The shortest route may include horrendous main roads.
The org's route will likely avoid them, and so will often be slightly longer than the shortest distance.
But there is the possibility that some riders might chose the main road anyway, so even if they do, that route must also be up-to-distance. Hence the shortest distance rule.
Sometimes, orgs may add info controls to force riders down the intended route.
Mandatory routes go a step further.
Sometimes, the *best* route is not practical for an organised event, for various reasons.
Perhaps it's so navigationally challenging that it's too hard to describe on a route-sheet, for example.