Here' some possibilities:
Windows updates,
program updates,
torrents,
background services on the other devices
Consider changing your password to make sure you're sure which devices are connected.
Many devices have options on them to track data usage, for example 'My Data Manager' works on ipads.
Maybe track your usage locally on your tethered devices. I don't imagine it'll be exact, but it shouldn't far off
The problem is theres tens of thousands of different possibilties, and there's no way to know exactly, unless you get someone that will locally track it.
-Alex