NY expressed precisely :
I presume manufacturers design the speedo to over-read by at least 2% so it
never under-reads even with just-legal tyres.

Yes, they make sure you can never get done for speeding, unless you
actually are speeding.

Mine seems to under read by 5
mph across the whole range from 10 - 70, compared with a GPS speedo app.

How they read, depends on how the calculate the speed - adding a safe
percentage or simply adding 3, 4 or 5 to the value. Mine uses plus a
percentage and the mph difference increases with increased speed. Last
time I checked, it was 73mph when doing a genuine 70mph from memory.