Your data usage is important. Many manufacturers design the software to work where if there is a slight break in wifi connection the mobile data will pick up and finish the data connection. This would explain why there is usage while on wifi. In order to prevent this from happening is to turn off mobile data. This will stop the data usage.
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I have never been near my data limit and I suddenly got a message that I am at 90% and I have nothing different in my usage from previous months. I am seeing things hitting my data overnight when I am asleep and clearly not using the phone and also on my home wireless and I do have the wi-fi assist turned off. Something is going on and I refuse to pay an overage fee!