Why "Access Point by usage" and "SSID by usage" total traffic is misalignment?

Zyxel_Melen

The "Access Point by usage" will record all the traffic on the AP, combine all SSID and the LAN port traffic(if the AP has the LAN port), therefore, the total traffic will be different to the SSIDs by usage, which record the traffic from a SSID of all APs in the same site.