Global CSV Export was added in 7.1. Why run such an ancient version? Doubt you are getting security updates! You should be on late 7.x release if not 8.0. Maybe you have old hardware? Time for an upgrade?!?!
DHCP uses unique combination of the MAC address and the Client ID. As an example, when using PXE boot, the Client ID is different, so the lease used by PXE will be a different IP than the Windows or Linux OS after the PXE boot.