[CentOS] NFS: hostname vs IP address

CentOS 5.7 x86_64. Lots of hosts NFS mounting a file system. All are configured identically (same LDAP servers, same DNS, same autofs config, same patches, etc). On some of them I see an NFS mount displaying a host name:

% df -P | grep smt <hostname>:/mnt/foo 1651345888 264620688 1386725200 17% /fs/home/smt

and on some just the IP address:

% df -P | grep smt aa.bb.cc.dd:/mnt/foo 1651345888 264620688 1386725200 17% /fs/home/smt

I cannot see any difference between the configuration of these two clients. Everything works. Anyone have a clue as to the source of this difference?

-steve