When riding in a timed event with my Apple TV, if I add the elapsed time and time left it is always less the the advertised time. Sometimes by several minutes. Yet at the end of the event the screen will show I have ridden the advertised time.
Hm, that does sound pretty strange. I invite you to submit a support ticket so we can look into this further. If you can provide a screenshot(s) of the problem, that would be helpful too.