Because the definition of a data session isn't as clear-cut as you might think.
Your device does not start a new data session each time you open a new app or visit a website. Instead, it opens an active data session which might have been active for some time already - up to a week is no exception.
As soon as the session has been closed, the system will process it and your usage and bundle will be up-to-date again. Such a session will usually close automatically, but you can also do this yourself by restarting your phone or by switching to the airplane mode for a few moments.
In order to help you monitor your usage, we added your open data session to the usage overview. If there is an open session, you can see how much data has been used so far. Do take into account delays of about half an hour in displaying such a session.