I am sure that you have, without a doubt, noticed that rebooting takes much longer than starting up from a shut down.

When you "shut down" your PC, it saves a copy of they system state / driver state to the HDD, and boots from that, bypassing starting up all devices and reloading drivers from scratch.

I don't know all of the technical details, but you can probably do a search.

This causes some confusion when looking at the Task Manager. If you look in task manager, under CPU, it might show that the system has been up for 8 days, which is misleading. This shows that the PC really doesn't "shut down". Sure, the power is cut off, but it is a kind of immediate hibernation.

The system up time only resets to 0 when you do a reboot, because all system data is discarded and the system will boot up from scratch, hence taking longer to get to the start screen.