How about a situation where user is telling you they are powering off the pc, but they just turn off the monitor and go home? 😁
If that devices has fastboot disabled, user is either closing the lid of the laptop, which default action is sleep/hibernate or the device has no LsAgent, in which case data may be off for minutes/hours depending on how often you scan your PC. It used to be that without agent, I wouldn't bother checking for uptime as it may be off. I would advise making sure that both hibernate and fastboot are deisabled and install LsAgent.
It also could be that your WU settings allow PC to awake for the updates, but due to PC malfunctioning, the Windows Update does not put the PC back to sleep/off state. I would advise to update Bios and than give it a check.