Basically, it’s because the off peak cap is £14.90 for both zones 1-6 and 1-9.
However, if you travelled in the morning peak the peak cap for 1-9 is £21.20 but the peak cap for just zones 1-6 is still £14.90
A paper One Day Travelcard would need to have validity in all zones 1-9 to be valid at Epsom.