Tom Tom route planning a little strange - Tours

Hi anyone, I am just trying to understand a route planning matter which I find strange. I am using a Tom Tom 700 and had planned an avoid toll route to Spain. On the section between Tours and Poitiers I expected it to take me along the main route 910 but instead it took me through Tours itself and out along the 751, 759 and then joined the 347 to Poitiers. Even when I tried to avoid the 751 it re-calculated the whole route and took me via Le Mans instead. Anyone any idea why, is the direct and main route national 910 not good for some reason? Regards, Roy