iOS 9 has a new feature called Wi-Fi Assist, enabled by default, which automatically uses cellular data if the connected “Wi-Fi network connectivity is poor”. While this feature sounds good in theory, it can result in a significant increase of cellular data usage without the user’s knowledge.
Prior to iOS 9, an iOS device connected to Wi-Fi would only use Wi-Fi, even if the Wi-Fi network was unresponsive. Cellular data would only be used if the iOS device wasn’t connected to a Wi-Fi network.
If you are running iOS 9 and have a cellular plan with limited cellular data usage, to avoid data overage charges you may want to disable the Wi-Fi Assist feature. Depending on your carrier, it can be found in either Settings->Mobile Data or Settings->Cellular->WiFi Assist.