What I've found is mapping/routing software and GPS software sometimes 'disagree' as to exactly where something is located. Just for grins, I went to 5 different online mapping sites and got the long/lat for our company 'address'. They were from 1500 yards to 3 miles off as to where we are actually located - and each one was different! You sort of get 'practiced' as to where to put waypoint to force the GPS to go where you want to go. Also, you have to really zoom in on the mapping software when you plant a point to make sure you are actually on the road and not a couple of hundred yards east or west.
For me. MS Streets & Trips and the PRIMO maps seem to usually line up pretty well but there is the time where they don't. If you find it wanting to send you back, press ROUTE OPTIONS>EDIT which will bring up the points of the route. Normally the first point is your current location. You can delete the 'erring' point (which should be the next one) and then continue on your way.