Why does it sometimes only show numbers instead of user names?

ftp 25191 0.0 0.1 5404 1336 ? Ss 16:10 0:00 /usr/sbin/vsftpd /etc/vsftpd/vsftpd.conf
515 25193 0.0 0.0 5428 868 ? S 16:10 0:00 /usr/sbin/vsftpd /etc/vsftpd/vsftpd.conf
ftp 25194 0.0 0.1 5404 1336 ? Ss 16:10 0:00 /usr/sbin/vsftpd /etc/vsftpd/vsftpd.conf
514 25196 0.0 0.0 5428 868 ? S 16:10 0:00 /usr/sbin/vsftpd /etc/vsftpd/vsftpd.conf
ftp 25205 0.0 0.1 5404 1340 ? Ss 16:10 0:00 /usr/sbin/vsftpd /etc/vsftpd/vsftpd.conf
519 25209 0.0 0.0 5428 872 ? S 16:10 0:00 /usr/sbin/vsftpd /etc/vsftpd/vsftpd.conf
root 25239 0.0 0.0 4036 724 pts/0 S+ 16:10 0:00 grep ftp

I think I may have figured it out, found this little tidbit online

"ps subsitutes the number representation for the saved user name when the length of the name is more than 8 characters"

and the users its showing the numbers for are indeed > 8 characters