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.