I can't rule that explanation out, but I think it's unlikely in this case: my DHCP server knows the MAC address of all of my machines, uses a static MAC->IP map

I can't rule that explanation out, but I think it's unlikely in this case: my DHCP server knows the MAC address of all of my machines, uses a static MAC->IP mapping, and doesn't update DNS (which is also static). The IP and hostname (as reported by `hostname`) are always correct; it's just the name as reported in the Sharing preference pane and in Finder from other machines on that network that gets the " (N)" suffix.