Of course, there's no real reason to go to 20 rather than 17 other than convention, just seems unusual
The first one shows how much time the person needed and the latter shows remaining time.
For example, for the current quiz it says for me: "Your fastest time is 3:37" and "Your best time is 2:19 remaining". Since I didn't pay attention to the timer in the beginning, I don't know if my current time is better than the previous try or not. For me it's more confusing than helpful.