Not entirely true, if it is a digital odometre then that is taken from the ECU, the speed and distance covered I have found on testing using OBD readers is way more accurate at the ECU, so i came to the conclusion that it is actually only in the speedo head itself.
I have tested this on 8 toyotas and 1 mercedes all have been about 10% out speed wise on speedo but pretty damn close on both speed and distance via ecu, this was checked using an APP on my phone and compared to GPS