So last moth I noticed that I used a extra 3gb of data on my bill. This cost me a extra $45 on my Verizon bill. So I started to do some research and found that on iOS 9 there is a feature called Wifi Assist. What Wifi Assist does is if you have a poor wifi connection it will boost that connection using your Cellular data. My personal iPhone is not on iOS 9 but my iPad is or was at the time. And was in the back of the house most of the time where the wifi signal is not to great. 

I am not saying that Wifi Assist is the culprit for sure but it is odd that I never go over or get close to using 10gb of data on my bill. And last month I used 13gb. So just something you all should look out for. If you want to turn off Wifi Assist, just go to your settings, cellular and at the bottom toggle off Wifi Assist.