I'm not sure if this is what you're looking for but from my personal experience, I think when you're using the Wi-Fi (if you're tethering from other devices) it will automatically download any software updates, such as apps, (which tend to use a lot of data) assuming this is the set up you have (unless you update the software/apps manually). Hence the reason for the difference between using the iPad on Wi-Fi vs. LTE.
You could check your 'Settings" under "iTunes & App Store", see if the "Automatic Downloads" are on and if the "Use Cellular Data" is on as well.
Hope this helps.
