It's not a rumour. They usually read somewhere between 5-10% over actual speed. Use a more accurate GPS speedometer on your phone to check that.
Have had a GPS speedo on the dash for a good dozen cars through the years and never seen more than a few mph off on a flat surface. That's something I actually noticed and looked for, for some reason. A few mph over speed is fairly common, but we're talking 1-2% at most. (confirmed with Tesla Model 3, Corolla, Fusion, Prius, Elantra, Mirage, etc etc).
A fixed 2mph difference at 20mph is 10% so imho they're at least _technically correct_.
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.