Schedule revising is such a mechanism, that it tries to fix the schedule, when the schedule is interrupted.
For example, your Raspberry Pi should be waken up on 2:00, but it turns on at 1:33. When 2:00 comes, your Pi is already on and the scheduled startup will be simply ignored. The bad thing is that, without booting on 2:00, Witty Pi's software can not properly schedule the next shutdown, which means the schedule script has been interrupted and will not continue.
Schedule revising is the way that Witty Pi software try to fix this problem, it detected that your Raspberry Pi is awake at a wrong time, and it immediately schedule a shutdown just 1 minute before the scheduled next startup time. By doing so, your Pi will reboot at the next startup time and the schedule script will continue.