Why does the IndoorAP use a different SSID and WiFi channel from the Hubs?

The answer is simple: so they can be successfully connected to an AyrMesh Receiver. If the IndoorAP had the same SSID as the Hub and the Receiver was able to "see" it, the Receiver would immediately connect to the IndoorAP instead of the Hub (and the network instantly would fail). It uses a different channel to make sure it does not interfere with the Receiver's reception of the Hub's signal.