Why does the clock reset without power loss? (No RTC battery exists.)

I have a Toshiba Satellite C55-B5298 that will have a "RTC battery low" message when returning to it after a period of time (overnight). The laptop had turned itself off and was turned on to see this message instantly. The laptop is set to never sleep while plugged in (only display sleeps). Hibernate is disabled, there was no AC power loss, and the main battery is installed. Toshiba omitted the RTC (CMOS) battery in this model. (I guess they counted on the user never removing the main battery.) The laptop has always stayed plugged in and the battery holds a charge.

What would cause the laptop to lose power long enough to reset the clock even though it stayed plugged in with a fully charged main battery?