I have found this:-

Suppose road A and road B run parralell a mile appart with fields in between.

I want to go to a location on A road and I use the post code to enter it. If the centre of the post code area is in the middle of the fields then the nav may take you to a location on the B road. hence all the routes and arrival times will be out.

If you enter the same destination using the 'cross hairs' it all works fine.

YMMV