Route Milage does not agree between Basecamp and my Nav6

I have made custom routes many times and while there is usually some discrepancy between the mileage I see in Basecamp and the mileage for the same route in the GPS (5-10 miles) which is understandable... on my new replacement Nav 6 I see DOUBLE the mileage although looking at the map on the Nav6 the map appears to be correct.
It seems as if the Nav is showing my route as a ROUND TRIP although the route directions do not show it is a Round Trip.

0 Peobody 8 hours ago

With the route showing on the map on the Nav, have you zoomed in a visually followed the route looking for anomalies? A route point in the wrong lane of a multiple lane road will add miles due to the u-turns required to visit it and then get back on course. Admittedly, it would take a number of them to "double" the mileage but that is the only troubleshooting suggestion I have.

A trick I use to verify that a route created in Basecamp is duplicated by a device is to created a track of the route in Basecamp and transfer that track to the device along with the route. I then show the track the map, load the route, and view the map. When there are no deviations, the route line will cover the track line. A visible track line indicates a deviation created by the device. My device is an XT but a riding buddy has a Nav6 and uses this same trick.