@firstname.lastname@example.org, the simple answer is that OC or it's contracted resource initiates the reduction early "because of the length of time it takes to send out the large number of signals necessary." Why, with the current technologies this can't be done in a matter of seconds is a different question.
There is, however, a very simple alternative. Since the TP-Link smartplug is programmed by OC to turn off and back on precisely on the hour, just put one of those plugs on the outlet into which your furnace is plugged. Of course you'd also want to disconnect the t'stat from OC. If OC was turning the set point down, this would be a less sophisticated method, but, since OC is just toggling the t'stats off and on, it works exactly the same.
I asked this same question a couple of weeks ago, but it was my fridge and freezer that were shutting off early. (They are on smart outlets). I have been on OC for some time and had not seen it happen that way before. I have actually come to like it though. It reminds me that the OH is about to start in case I have not been on email or noticed my text reminders. I have notices that, both for shut down and start up after the OH is over, not all the devices go off or come back on at the same time.
My observation last night is that my AC unit was turned off when I received the TXT message, 5 mins before the event. As soon as 6pm clicked over, KASA was showing my 4 smart plugs had been turned off. OhmConnect was showing they were still turned on and it took 10-15 mins for this status to be updated in the devices tab.
My Ecobee3 (Thanks for the rebate again!) Shuts down around 20 minutes before the OhmHour occurs as well. My TP-Link devices that work shut down promptly on the hour most days.