itunes account on iOS 8.3 used up a lot of cellular data even when you are in wifi

Hi,

I have a problem about the cellular usage of my iphone 5s with iOS 8.3. It happened several times. The itunes account uses up a lot of my cellular LTE data even when I am using wifi for my phone. for example, yesterday between 2pm and 3 pm, I was in my office with wifi on. the itunes account used 289 MB from my cellular data, while at the same time my wifi usage was about 1 GB. because I was using wifi, I thought all functions of the phone should use wifi. But from a app, it shows there are always some services using cellular data at the same time when the phone is using wifi. usually, the cellular usage is small, but this particular service, itunes account, will use a lot cellular data once per several days. I checked all my settings with itunes & app store, music, I have turned all "use cellular data", "automatic downloads" off, and I donot use "itunes match" or "genius". for the Podcasts, I turned off "All notifications", "Bachground app", and "Use cellular data"; I leave "Sync Podcasts" and "Only Download on wifi" on; I set "Refresh" manually, "Download Episodes" off. For ibooks, I turned off "Use cellular data". I have turned off "icloud photo library“, and "my photo stream". I have tried all I can to turn off the cellular use of any function relative to itunes account. But it seems the "itunes account" is still using up my cellular data.

I am thinking to have an ability to turn off the "system services" to use cellular data. will it be a function in next version of iOS?