I have a iPhone 6 (iOS 8.4). My problem is not that the data usage shown in settings/cellular is extremely high but rather that the sum of the usages of the apps and system services shown below is not equal to the total usage at all. There are differences with more than 200%. Do you understand what my problem is? Please do not tell me that's normal and this is background usage. Those background usages would also have to be shown up in the cellular settings.