There are a few instances why this happens:
- It truly depends on how an app classifies data. In some cases where the subscriber is viewing content that is hosted outside an app (i.e Facebook) it may be classified as third-party (i.e non-facebook) data. The usage of third party data is likely the reason why your base data is depleted.
- A non-Unlimited App is running in the background.
Please ensure that your Unlimited status = OK when you browse Unlimited Apps.