Libvirt

Hi All, I am trying to collect memory, disk and network stats for a VM on kvm host. It seems that the statistics are not matching what the OS inside the VM is reporting. Why is this discrepancy ? Is this a known bug of libvirt ? Also I heard that libvirt shows cumulative figures for these measures ever since the VM was created. Also I tested by creating a new vm and comparing the stats without a reboot . Even in this case, the stats dont agree. Can someone help me here please ? Thanks Kumar