Does the iPhone 5S with iOS 8.4 have a cellular data addiction? Recently, I've been running out of data each month and it's being bugging me because most of the time my phone is either at home, or at work, where there is WiFi. Surely I can't be using that much cellular data I thought.

So I ran a test. Connect my phone to my home WiFi, I reset my cellular data usage statistics to 0 and then dropped into the AppStore to download an app (any app, it didn't matter which). Re-checking my celluar data statistics straight afterwards - 12.3Kb used. My question is why?

It's important to note that on my phone cellular data was turned on and I had an E connection at the time. Background app refresh was turned off. During the test, I didn't let the phone lock or go into standby mode, so there shouldn't be an issue with the WiFi going into hot standby. The WiFi was active the whole time (I was using it).

I know 12Kb isn't a lot but I'm chasing a principle here. My WiFi is on and being actively used - so why is my phone still using its cellular data connection? What else is it doing favours a significantly slower connection?

Can anyone help me debug / trace this? The best I can find is that "System services" is using a small amount of data (0.3Kb), mostly attributed to push notifications, but beyond that I'm stuck.
There are a lot of situations in which your phone, while connected via WiFi, might need to make a cellular connection. This would include normal communication with a cell tower, incoming calls, Visual Voicemail, or depending on your WiFi signal and performance, additional data from apps.

Once you have reset your cellular data usage statistics, re-run the same test you did. Once complete, go down into Settings -> Cellular  and look at which apps now report data usage. This will help you narrow down which app is causing it, but again there are too many potential answers as to why it is happening.