Specificity of Data Usage on iPhone
As it stands it shows the data with a (±) 50 Mb resolution.

You can see it as fine grained as per application data usage.

If you scroll down further (in Cellular Data usage), you will get an alphabetical list of all the apps have use cellular data.

If you scroll all the way down at the bottom, you will see a button called System Services. If you tap on that, it will show you how much cellular data you are using for things like Messaging Services, Push Notifications, Documents &amp; Sync, iTunes Accounts, Siri, Voicemail, Software Updates, Find My iPhone, Diagnostics and Apple ID Services.