Your pf.conf does not have a redirect for port 1720, only for port 21. Port 1720 is not being serviced by your OpenBSD system, as far as I can tell.
Port 1720 is used for
Microsoft NetMeeting. Are you absolutely
sure you are scanning the correct physical device? If you are testing connection to the OpenBSD platform via another router, you could be seeing a port redirection:
Code:
[nmap testing system] --- {NAT router} --- [OpenBSD]
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[Windows system]