It' not really a mysterie, unfortunately. It's called "Modern Sleep" or "Modern Standby". It basically means that the OS uses this time without user interaction for syncing lots of stuff. Windows is particulary aggressive about this, which makes laptops sleep and battery endurance without disabling WIFI impossible to calculate. The battery drain including heating up the machine reliability is real and severly compromises reliying on battery power while on the move. In windows world this even fried laptops in backpacks.
The best and only true solution for for laptops running Windows and MacOS is to disable any network connection, that is WIFI when on the go.
Don' waste time to guess who is the culprit. The main responsibiloty for this behaviour lies with the OS.