I wonder if fast startup is messing with my uptime statistic. When you shut down with fast startup enabled (which it is by default), it puts the system in a kind of hibernation where it writes the kernel state directly to disk so that the kernel doesn't have to boot up from scratch when the computer boots.
I would guess that installing Windows Updates temporarily disables this because the updates modify the kernel and probably require the kernel to boot from scratch after the updates are installed. However, the last time updates were installed on my system was on Feb 21, but Task Manager says the computer has been running since Feb 28.
@Burgi USB devices could be waking it. My mouse randomly lights up when the computer is off (it is gaming mouse with fancy lighting).