If there is nothing wrong with the source cartography, then don't change anything. Probably the car isn't fixing you exactly to the road. This where my Mib2 high flags up 6mph at the bottom of the road. I just ignore it. Thinks I'm off road. It use to be fine.
There was a way in mib2 standard I think to calibrate the sat nav better. Mib2 high relearns where you are. When I've messed about with my mib2 high unit in the developer's menu, it looses tract of where it is. It re teaches itself as you drive down the road. You start of, off road but after a few turns it pulls the location back on the road.
It could be within mib3 there is a calibration setting in developers menu, not something I've looked for since I don't have mib3 or the people that process these files for VAG have an element of rounding error going on. Of course the rounding error could be going on inside the unit. They use a range of different satellite providers, this is where in Obdeleven / VCDS you can change the boutique of the satellite group in mib2 high at least. My mib2 high is only on the traditional US ones, mib2 standard newer ones use US, Russian and Chinese ones... mind you for mib2 standard they still don't get the turns right on roundabout.
It could be your aren't getting a very good signal at that point. I've seen mine take a wander in MK as if being jammed, and I've been in Jordan where it was being jammed for real. Turned round and went back. Too close to the Syrian border. That was using the US constellation.
So it could be the hardware / need calibration.... failing that the people that process the source cartography for VAG haven't zoned speed to the road segment but an area. There was an early issue in MIB3 where postcodes were fixed to points other than road segments so it sent people off road. Could be naff programmers that don't understand GIS. That was my belief before, but it did get sorted out. The road speeds may not be if they allocate them to a zone with rounding error rather than the segment.