It is because of the logic that how our system tells if a device is offline.
Take a Zigbee device for example:
It usually sends a status report to the gateway every hour. If the gateway misses one of these, you’ll get a warning that the device is offline. But remember, there’s an hour-long gap between these reports.
Wi-Fi devices work similarly. If one keeps connecting and disconnecting or your router decides a channel switch, you’ll get flooded with notifications on your phone and our server. To avoid this, a preset duration is needed to confirm if it’s really unreachable.