I know it sounds like a lot but in my experience the difference is mostly a fixed offset plus a tiny percentage due to tire pressure/size.

A fixed 2mph difference at 20mph is 10% so imho they're at least _technically correct_.

5% at 70 mph is 3.5 mph

10% at 30 mph is 3 mph

I saw this with various European cars.

My experience is that it seems to be a fixed percentage rather than a fixed amount i.e. the absolute difference increases with speed.