Remember that what gets displayed on the dashboard need bear no relation to what the vehicle actually "knows" about its speed. With electronic displays it's just a question of selecting a number from a table, take your pick.
Also there is no reason for the speedo and odometer to bear any significant relationship, they come from different calculations, one is total number of counts (odometer) and the other is a frequency (speedo pulses). One might be 5% optimistic, the other might be 4% pessimistic, it's whatever the equation/tables say. It's not like the old analogue instrument driven by a rotating cable, though even then the marks on the dial could be put anywhere the designer liked......... and it was a question of how wildly the needle would swing back and forth.