BC – Average consumption completely wrong values at 1.6D

Hello, with my V50 1.6D MJ 2008 the BC constantly shows a much too high average consumption. I fill up full and then calculate the actual consumption. After fully refueling I always reset the BC completely. The deviation is the greater, the more I drive in the city. Sometimes I drive a whole tank filling away in the city, then the BC shows values between 8-10 l/100km. When calculating then I arrive at 6-7 l/100km (average consumption). The BC shows then already an average consumption of 2.5l at times! Only if I drive only highway and country road (holidays + departures), the indicated average consumption reaches the actual value. Then the BC shows still about 0.5 to 1l too much, but nevertheless. Unfortunately on the 26tkm driven so far an average deviation of 1.2l/100km has set itself. At the first inspection the friendly one has made an update, without effect! The tyre measurements were also checked. With my old V40 1.9D MJ 2001 the BC values were very accurate, but not so much to the average deviations?