While I have absolutely no experience with DNS on Win2k, I do know that with bind there are several ways to set up the DNS records for a certain zone. Let me show you my example and it might then make sense...
Lets say you are surfing and you ask to go to
www.coolsite.com. You computer then figures out which DNS controls the coolsite.com zone. It then asks that DNS server, "Whats the ip address of www host in the coolsite.com zone. The DNS server then sends back the IP # so that the actual request can now take place from the server that is hosting
www.coolsite.com.
Now in the case of your hostname. All your mac knew was the IP# your internet interface(nic card, modem, etc...) So in order to find out what hostname, your mac has to do a reverse lookup. But here is where it gets a bit different. Lets pretend that you have assigned 199.45.191.25 to your NIC card. Since it doesn't know your hostname, it has to use your ip number to figure out your hostname. Instead of figuring out which dns hosts the coolsite.com zone, it has to figure out who hosts the 191.45.199.in-addr.arpa zone file(which is probably the same DNS server as above, but doesn't have to be.) If you notice, the numbers I listed above in a reverse order to your pretend IP #... Why they do this, I don't know but thats the mechanism by with reverse DNS lookups happen. What I am trying to impress upon you is this... there are 2 separate zone files that reference your IP# and hostname, the forward and reverse.
As for Win2k, since you had a reverse zone enabled, it probably used it. After deleting that zone that said cujo, Win2k DNS probably used another mechanism to reverse the ip to your actual hostname. Maybe someone else with Windows DNS experience can chime in here.