Mine seems massively random and I just completely ignore it.....
In my camper van, where it is more difficult to see the speedo, I have just bought the Snooper Myspeed XL which shows your speed and the speed limit of the road you are on, based on GPS/mapping data.
It is far more accurate, but even so not 100% so.....
There doesn't seem to be any substitute for looking out like a hawk for the posted speed limit