@denis it’s mostly a technical reason. The retention of 7 days is on the data, but we also store the number of stored messages per device per hour and this data has no retention. We don’t display this now, but the idea is that we might show you device activity per hour in the future, with a longer history than 7 days.
Summary: there’s still the device in the database, but not its data, only its number of message per hour.