NX-OS uses a random MAC address from the IEEE OUI block that is assigned to Cisco.
The VDC system MAC is pulled from the active supervisor and concatenated through a hash to ensure uniqueness.
NX-OS uses a value that is based on a range that is assigned to the SPROM of the system backplane.
The VDC system MAC is based on a range that is allocated to the first line card that is installed in the switch.