Welcome to the wild wild world of GPS and push button routes! I don't have a Tom Tom, but have been using a Garmin Nuvi for several years and recently used it to go across the USA from North Carolina to California. Where they shine is getting back to a location AFTER you have gotten yourself lost. For planning routes they are useful, but not very trustworthy (as you have found out). Every year here in the states there is a sad story of someone (or a family) that gets routed onto a long lost road and ends up dead or with hypothermia because they trusted their GPS and not a printed map.
My GPS often tries to route me off an interstate, just to drive a parallel road for miles, and then get back onto the same interstate....go figure. It tries to send me on the strangest routes to get to a destination, even after I use the computer to set up a particular route. It has had me make repetitive right turns forming a perfect square to get back to the route I am on.
At times it can be useful to route around a traffic backup or construction. For a technology that should by now be almost perfected, it does leave a LOT to be desired.
My riding buddy that went across country with me has the motorcycle version of the Garmin ($$$$), and between the two of them, we only got off route a couple of times. Those times we used paper maps (A US Road Atlas) to get back on track. Even though we would program them both with the same destination, they very seldom took us on the same route. If anyone knows a GPS that WON'T get you lost and will always follow a preprogrammed route, please let me know.