IMAP/POP3 processes with numbers as users

Recently I tried the ps aux command in my server and noticed some imap and pop3 commands with linked to numbers as users. As you can see below. 549 12903 0.0 0.1 43528 3416 ? S 13:00 0:00 dovecot/imap 525 12904 0.0 0.1 43512 3380 ? S 13:00 0:00 dovecot/imap 525 12911 0.0 0.1 43524 3548 ? S 13:00 0:00 dovecot/imap 549 12915 0.0 0.1 43512 3400 ? S 13:00 0:00 dovecot/imap Do you know why?