Recalculating Custom Routes, Yes or No?

[Peobody]

An unusual prompt showed up on one leg of one of the trips, stating I was off route and wanting to know it I wanted to recalculate.
It was in the middle of a five mile stretch of interstate with no exits along to whole length I was traveling. No other roads nearby, either. I selected NO because the map was displaying the route correctly, and it then continued to follow the planned route for the rest of the trip.
I've seen this happen before on custom routes, even on routes planned totally with Basecamp alone. Not sure why it does that.

My XT does that as does the Subaru satnav in my car. I attribute it to position detection being off for a few seconds but I am clueless about what is actually causing it.