I'm guessing it's more likely that this data, along with other setup data, is stored in sram ready to be copied at an instant's notice into the flash when the soft power button is pressed to initiate the shutdown process controlled by the OS. It may be being saved to sram every 60 seconds but this only gets written to the flash on each soft power-down cycle.
Of course there's always the alternative strategy of triggering a short (10 to 30ms?) write to flash cycle when the PSU PG signal is negated by the loss of mains supply utilising the few hundred milliseconds worth of reserve power remaining in the HT smoothing caps to complete such a short write cycle to guard against even this most extreme of shutdown scenarios.