Hi
A possible way would be to add a Client ID (an appropriate name for example) in the Client ID field on the macs concerned. Yet another way would be to create static maps for each mac as well as A Records mapping the reserved IP to the created name. Unlike PCs, Macs are DynamicDNS aware on the Reverse Pointer. In other words they won't necessarily append the domain details handed out by your DHCP Service to their specific computer name once they get an IP Address. And it won't necessarily show up in your DHCP Service either. Since 10.5 all Macs will do a reverse PTR check on the given IP address and then assign themselves the name associated with that IP address which may not necessarily be their own computer name. It may well be a stale record left over when the IP address was previously assigned to a PC. In my experience this is what shows up in the name field in the WINS tab. If you launch Terminal you may also see it appear there as well.
The name you assign in the Computer Name field (System Preferences > Sharing) does not define their hostname. It only defines their Bonjour/Rendezvous or .local name. To define a hostname you'd have to do this either in your DNS Service (as already mentioned) or use a combination of DNS and the command line utility "scutil"
Something like:
sudo scutil --set HostName name.yourdomain.com
Of course you'd still have to create an appropriate entry in your DNS Service for this to match up with the given IP address. If IP addresses are constantly changing you'd have to update DNS accordingly. Hence the reason to create static maps. That way DHCP will always dynamically assign the same IP address to the same hostname.
HTH?
Tony