Laptop battery is discharged even when laptop is turned off

I have a new Lenovo laptop. When battery was 100% i turned the laptop off. I did not power on for about 20 days. After that , i turn on the laptop and i've seen that battery was 1%. I've contacted with lenovo and they says that if the Fast Startup option is enabled , this is the cause of discharging the battery because even when laptop is off ,Fast startup requires power. But i'm not conviced with this response ,because what about a desktop pc that has Fast Startup enabled , and when it turn off and disconnected from power ,there's no battery at all , and how Fast startup works in this case ??

Question by alex on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.