I'm also having the same problem - I'm obviously missing something silly. Any hints appreciated :)
I've been using OpenDHCP for a few years now and it still doesn't report the correct number of available leases. One of my IP ranges is xx.187.56.7 to xx.187.56.11 and OpenDHCP reports 4 available leases whereas the actual number is 5 Another range is xx.187.56.14 to xx.187.56.55 and OpenDHCP correctly reports 32 available leases. Not sure why the first range has never reported correctly