there’s a default in your iOS 9 settings that you might not be aware of, and you may want to switch it off.
“Wi-Fi Assist” is an option available within your “Cellular” settings. it uses your data for support when your Wi-Fi signal is not very strong. this makes all of your iPhone’s web content flow more smoothly. it makes video buffering a nightmare of the past. music-streaming services deliver non-stop playlists when you’re on the move. websites don’t time out when you’re browsing-while-walking from, say, your office Wi-Fi network and stepping outside to lunch. your cellular signal kicks in as the Wi-Fi weakens (and eventually drops your device when you’re out of its reach).
so, why would you want to turn this great feature off? why choose anything other than optimal performance?
there’s a good chance you’re using data even whilst connected to Wi-Fi.