Hello Tomasz,
The BSSID is based off of the AP radio mac. When more then one WLAN (BSSID) is mapped to the radio, the BSSID mac increments by one.
If the AP radio mac is 00-23-68-2E-78-C0, the first WLAN mapped to the radio would use this mac address. If you mapped a 2nd WLAN to the radio, the BSSID would broadcast as 00-23-68-2E-78-C1, the 3rd 00-23-68-2E-78-C2 and so on.
Hope this helps.