There is a growing issue that has existed for some time of iPhones using cell data while the device is connected to Wi-Fi. After experimenting with several iPhones, the evidence is clear that the sleep mode function of the device is causing the issue.
When you use your iPhone with Wi-Fi, it does use all data services via the Wi-Fi. Most people when using apps, simply just press the home button to get out of the app. They do not log off the app or kill the app by double clicking the home button, and pressing down on the app's in the lower field, and killing the apps by pressing the do not enter badges that appears. This will stop the apps use of cell data, and Wi-Fi.
When the app isn't logged off or killed. The app's still run in the background. Most people after use, simply lay the device down, and go about their business of life. Everyone knows that the device will eventually lock the screen which is located via settings app > General> Auto Lock. Whenever the auto lock is set, the device will go into sleep mode. When this occurs, the device will disconnect itself from Wi-Fi as a battery conservation program written in the source code. During this mode, any app that is working in the background, or any update by the device itself will use cell data.
This issue becomes more obvious when users have several devices connected to the Wi-Fi network in the home or office. Because of the amount of bandwidth that is being used, sometimes the iPhone will not re-establish its connection with the Wi-Fi. This is even more apparent when the distance of the device from the Wi-Fi source is more than ten meters (32.5 feet). After testing several iPhones over the last several days, this is a problem that is costing users 100's of Dollars, and Cell providers Millions of dollars in credit to their consumers.
This problem is resolvable. Android devices already have a switch that allows their devices to remain connected to Wi-Fi during it's sleep mode.IPhones can do it with a simple software update.