As John NY was saying, this is not an error. BMW intentionally is showing a higher speed. I remember on older BMWs the OBC was showing the true speed and the needle was showing true speed plus aprox. 3%. On F10 (at least mine) both are showing the same speed, higher than the actual.
2012 w204 Mercedes-Benz C350 4MATIC MY 05/2012 Black/Sahara
2011 F10 535i xDrive MY 01/2011 Sophisto Grey/Black