On a lighter note, a few years back, for a few years when I rode the " The Red Rose" ride from Halifax, when I got to the finish, I would be moaning about how hard the hill is at some point, about 15% to 20% with about 3 tight hairpins. After about the second or third year, the organiser said, "You should not be going that way". Sure enough, the next year, I still went up it and could not really work out why as I was following the route on the Garmin.
Eventually I worked out why, my Garmin was set to avoid "A" roads and at a certain point, it recalculated my route to avoid an "A" road, but in doing so, took me up the hill and then I rejoined the route again on the "A" road as there cannot have been an alternative for the Garmin to calculate.
Anyway, I resolved the issue by using GPX files for all Audax rides, the type that a Garmin will not recalculate and taking more notice of the route card.
The morale of the story is, perhaps the odd rider sometimes varies off route because they have had a similar issue and not known themselves they are taking a deviation.