Does the iPhone 5S with iOS 8.4 have a cellular data addiction?
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.