Have you ever deleted any users from the system before? Linux/Unix use UIDs to identify users, and they can get reused...
Or, perhaps you've untarred a tarball somewhere on your system (either before or after you created the new user) which had files in it owned by the same UID your new site user uses.
try this:
# find / -user username
replacing username with the name of the new user.
It will give you a list of all the files owned by that username.