On-time / late dot colour information?

Hi. On the new iOS app there are green or red dots near the actual and estimated times of a flight on the information panel. I've noticed green dots against flights that are technically late, so there is obviously a leeway built into this. I can't find the particulars of this leeway described in FAQ or help. Could you point me in the right direction to understanding when the red dots pop up, please. Regards, Dave S