Check Data Consumption If Wi-Fi Is Enabled in IOS 9
Wi-Fi Assist is a cool little feature in iOS 9 that switches your phone to your cellular network when your Wi-Fi connection is down. Sadly, some users, including Quartz’s Mike Murphy, report that they are eating up their data plan.
Wi-Fi Assist essentially turns off Wi-Fi and switches to cellular when you have a bad connection. This usually happens when you are on the edge of your Wi-Fi network, for example, if you are standing outside your home or office. Do this enough and you will begin to see it in your data consumption. To check app usage, go to Settings> Cellular to see data consumption (you can also go to your cellular provider’s website for general usage information). If this is too much, you can turn off Wi-Fi Assist simply by scrolling down the page.
You can turn off this new iOS 9 setting if you don’t have unlimited data | Quartz