This is usually a result of having different selections in the setup. One unit could be set up to navigate based on shortest distance and the other set up to navigate based on shortest time. It could also be what type of features one has selected to avoid in navigation such as avoid highways. Different maps can also produce slightly different routes as well, such as someone might have the Garmin maps and another unit has something like Openfietmap or some such. Unfortunately, modern items can make things easier as well as making them complicated.