Meaning of Time Machine configuration keys Interval, Delay and GracePeriod
From https://www.baligu.com/pondini/TM/13.html and based on the default values of Interval, Delay and GracePeriod one could conclude that:


GracePeriod (with default of 30 minutes) determines the following effect: Restarting your Mac will reset the TM schedule, but it won't run a backup immediately, even if it's more than an hour since the last one. It will wait about 30 minutes, to avoid conflicts with other things that may run automatically, or manually by the user, after startup.  
Delay (with a default of one hour): When your Mac wakes from sleep, it will do a backup immediately, unless one was done within the previous hour. That will reset the schedule.
Interval (with a default of one hour): TM will start the next backup an hour after the start of the previous backup, if your Mac is awake and the backup drive is available.