Are you getting 'Wheel Speed' indications in the diagnostic screen? If not, then this will show your position as being out when compared to reality.

Had this issue in my E46 where I was always ahead of the map pointer by 20 oe 30 meters - the dealer had used the correct full retrofit kit when they installed my nav, but some of the looms were faulty from the factory and had the wheel speed sensor wire in the wrong connector. As soon as it was put into the correct socket, the accuracy was spot on. This speed signal, plus the input from the accelerometers is used to 'correct' any accuracy issues with the GPS and to keep the map updating correctly when you are out of GPS coverage such as in a tunnel.

It may be that there is a fault with the speed sensor line, or the loom is bad/faulty/miswired.