Ethernet MAC Address Changes from ESX Server or GSX Server Assigned Value to VirtualCenter Assigned Value (1968)
VirtualCenter reassigns MAC addresses by design. Doing this ensures that all addresses within VirtualCenter control are unique. This automatic conversion cannot be stopped.
You do have the option to mark the address as static in the configuration file, though. VirtualCenter does not change addresses that are marked as static, regardless of address range.
For related information, see Setting a static MAC address for a virtual NIC (219).