Apple explaining one of the many reasons OS X server sucks
from bootpd(8)
The server tries to give the same address back to a client by remembering the binding even after it has expired. The server removes an expired lease entry only when it runs out of addresses, and needs to reclaim an address in order to fulfill a new request. When the server receives a DHCP Release packet, it sets the expiration for that lease to now, so that it can immediately reclaim the address if needed.