The reason is as follows:
In iOS 9 function Wireless Assist is activated automatically. This option is intended to offset a slow connection Wireless by the simultaneous use of mobile data. Is your Internet at home so slower than Apple addresses it, then the mobile data to be automatically activated in order to obtain a certain speed limit.
That is why we recommend you absolutely WLan Assist on the iPhone with iOS disable 0 to conserve your mobile data volume and to refrain from unwanted costs. And so off your Wi-Fi on the iPhone with iOS Assist 9:
1. Open Settings -> Mobile network
2. Assist Disabled WLAN using the controller
Should you own a iPhone4, then you will the function can not find because wireless Assist is only possible with an iPhone with 4G / LTE connection.
You know now why your iPhone with iOS 9 more data volume required as it was the case in the previous version.