File ownership in UNIX is determined by the numeric user id (UID), not the user name or login id. In the password file, each user name is associated with a UID. With rare exceptions, the UID is unique to one user name. When sharing files through NFS, it becomes very important to maintain consistent UID assignments to the same person on more than one system. For this reason, ITC long ago began assigning UIDs and keeping them associated with user names in the whois database. So that we can maintain consistency of UIDs, ITC does not reallocate UIDs when accounts are deleted. People do leave the University and then return, so we retain the name and UID association so that we can reactive the old ids. Since 1999, we have been allocating UIDs which are larger than 16 bits in length (greater than 65535).
Many UNIX implementations support UID lengths of 32 bits (maximum value 4294967295) while others, most notably Linux, still only support 16-bit UIDs. With the 2.4 Linux kernel (e.g., RedHat release 7.1), Linux promises support for 32-bit UIDs. You must be very careful when sharing files between 16- and 32-bit systems if you have any UIDs which are greater than 65535.
Don't just arbitrarily assign UIDs on a 16-bit system to try to map the 32- to 16-bit quantities. To accomodate the relatively small number of people who must do this kind of file sharing between 32- and 16-bit UID systems, ITC reserved a group of UIDs in whois to allow us to reassign individuals' UIDs as needed. Contact accounts@virginia.edu if you require a UID reassignment to allow this kind of file sharing.