I’m not familiar with the Overlander but em general that might happen for 2 reasons:

1- you are following a route (not a track or a course) and the map installed in your device is other than the one you used to create the route on Explore.

2- you have recalculate set to on.

in order to assure you are going to follow the exact same you have drew you have to follow a course or a track and have recalculating set to off.

I was not able to paste a web link in here but search for this : 

What are Tracks, Routes, Waypoints, Activities, and Courses on Garmin Outdoor Handhelds?