revert to what? the link you specified will get it back to the factory image that requires the Key to be entered. We dont offer the older images, there are kernel changes and rc changes that are needed for safe operation.
it can depend on boot order/update order etc
say you turn it on, let it boot, then reboot for the update. the nvram in the radio modem will have settings leftover.
if you had it off, then turned it on straight to fastboot, then the modem would never get those settings, and never remember some of the old state.
i think this is why every now and then we see the need to double flash the unit. its just a case of a tiny bit of leftover state. again this is my best guess.