Dann würde jede SSID eine eigene MAC Adresse aufweisen. Dem ist aber nicht so.
Das hat mit MAC Adressen auch nicht viel zutun... Es geht um den overhead den jede SSID bzw. VAP erzeugt, der Airtime „kostet“.
Hier ist es ganz gut erklärt:
Consequences of Multiple SSIDs
One frequently overlooked aspect of wireless networking is that a network administrators can control interference and channel utilization generated by their own managed wireless system. Dashboard allows admins the ability to enable multiple SSIDs on a single physical AP (Access Point). Each SSID that is enabled on a given AP is called a VAP (Virtual AP). VAPs behave as their own independent AP, operating on the channels the physical AP is set to. Therefore enabling 5 SSIDs on a single AP in Dashboard is nearly identical to deploying 5 physical APs with one SSID each. Normally, multiple SSIDs are used to provide different types of wireless network access to different device types and user classes. The downside of enabling more SSIDs is that it creates more channel utilization due to overhead.
Overhead from Management Frames
Beacons and probe response are two types of required wireless management frames that can increase channel utilization. Beacon frames are used by the VAP to advertise the SSID and inform connected clients that frames are waiting for delivery. Each VAP must send a beacon every 100ms at the lowest supported data rate so all clients can receive it. The date rate is 1Mbps by default with 802.11b/g/n and 6Mbps on 802.11a/n.
Wireless clients can also discover available wireless networks using probe requests. When a VAP receives a probe request, it will respond with a probe response for the the SSID which contains the wireless capabilities. Probe requests and responses are always sent at the lowest supported data rates with 1Mbps 802.11b/g/n and 6Mbps on 802.11a/n.
As the number of wireless networks operating on a specific channel increase so does the amount of beacon frames and probe responses. Take a scenario where there are two physical APs on the same channel each with a single SSID. Both APs will transmit one beacon frame every 100 ms and when any client sends a probe request on that channel, each AP will send a probe response. This would not cause much overhead. However take the same two physical APs each with 4 SSIDs. Now 8 VAPs are independently sending beacon frames every 100ms and any time a client sends a probe request, 8 probe responses are transmitted. This example does not begin to take into account neighboring WiFi system management frames, wireless data transfers, or non-802.11 interference (such as microwaves and cordless phones).
