[Discuss] Standby battery drop: Airplane WiFi (50%) > WiFi +4G (3%)

I found airplane mode with Wifi on consumes a lot of battery (standby with screen off).
Below is the battery drop of one night (about 7 hours), using the latest official rom [G920FXXU2COH2] for G920FD.

- Case 1
Turn the phone into airplane mode and turned on the WiFi. About 50% battery drop for one night.

- Case 2
No airplane mode. Both WiFi and mobile networks are connected. About 3% battery drop for one night..

I don't know why this happens. Can anyone confirm that and tell me the cause to the battery drop. Is this a baseband problem?