[libvirt] why mac are different between inside and outside of vm

hi, all nwfilter has many rules which depends on mac of vm, but i find the mac address inside vm is different from mac outside of vm. outside mac: vnet0 Link encap:Ethernet HWaddr FE:54:00:71:15:7B , inside mac: eth0 Link encap:Ethernet HWaddr 52:54:00:71:15:7B ,virtio why? i am afraid that nwfilter use the wrong one. how nwfilter chooses mac? and what is the usage of vnet0's mac? thanks very much.