Apple iPhone users: if you have updated or have a new iPhone device with the iOS 9 or 9.1, its feature Wi-Fi Assist automatically switches your device from Wi-Fi to cellular signal when Wi-Fi is weak, therefore your data usage will increase.

It is recommended you turn off this feature to minimize your data usage.

Instructions for turning off the WI-Fi Assist Feature are as follows:
Go into Settings.
Choose Cellular.
Scroll down to Wi-Fi Assist (close to bottom).
Switch the slider to the "off" position.