There are three main reasons why your data may be changing overnight or for dates in the past.
The Last 24 hours
Access logs are typically available only after a session has ended, implying a delay of up to several hours for session data to become visible in historical analytics. Consequently, the amount of data available in historical analytics can be reduced if a session has not yet ended/expired.
Multi-Day Listening Sessions
There are instances where a listening session may persist for multiple days or weeks. As the access logs are only available on disconnection, this data will be retroactively updated.
This only applies to "All Sessions" as sessions longer than 24 hours will be flagged and removed from "Qualified Sessions" This change is intended to better reflect human listenership.
Approximation Functions
The approximation functions used to derive user and session count may provide different approximation results when run on the same dataset.
The guarantee when using approximation functions is not that data will be exact, or remain exactly the same, rather the guarantee is that the error factor will be no larger than 1%.