Is it possible that it's trying to route you around traffic jams, or construction?
Then there's the little problem my wife had with Googley-maps on her phone. It was telling us to turn our car onto a greenways/bike-path sort of thing. She had it set for walking directions, instead of driving. 
Nope. Rural locations. Three different times, on trips to three different locations, I've had the GPS have me drive the two long sides of a triangle when the logical route (in both shortest time and shortest distance) would have been to stay straight, on the main road. In two of the three instances, the GPS took me a mile out of my way when the point where I came back onto the main road was within easy eyesight of the point where the GPS had me turn off. They can be useful, but they cannot be trusted. As Ronald Reagan said, 'Trust but verify."
How you verify when you're in territory you don't know (which is why you use a GPS) is a mystery to me. Maybe you need to run two (of different brands), and if they don't agree on what to do next -- stop and assess options.
