The computer that is "off" (not hibernating) will not use any data but if a router is connected a little further up the "data chain" other devices can use data like mad.
Example: My Nook HD+ was in a soft-off state. The little beast woke up in the wee hours and gobbled up 108 MB, then slipped back into a soft-off state.
You simply can't trust that you successfully changed all the setting for all the apps on such devices.
You really have to watch other settings like the BIOS setting for "wake on LAN".
Then there is the subtlety of Windows update settings .... sure it won't install updates until a specified time but ...... BITS can run like mad in the background gathering (downloading) that data.
BITS is also a favorite target for malware to attack.