I own a couple of Garmins and I've noticed that sometimes the route it computes going out isn't the same as the route it computes coming back. I also have a TomTom (or had one) and the routes it always calculated seemed to be based on random numbers and the phase of the moon.
Anyway, other than ignoring the route and going the way you want to and putting up with the constant chatter of recalculating and u-turn directions -- just drop in a couple of waypoints along the route you'd like it to take. That usually works for me.