nmap gives info not of open ports but on services that are running.

[Strong]

I have noticed that nmap gives me info not of open ports on an
interface but on services that are running.

$ nmap 192.168.0.1

Starting Nmap 4.11
Interesting ports on COR (192.168.0.1):
Not shown: 1678 closed ports
PORT   STATE SERVICE
21/tcp open  ftp
80/tcp open  http

While in iptables all ports are denied. And a connection can not be
established to that interface till I allow it in iptables. Why is it? Is
there a way to know the open ports - the ones I can connect to, meaning
they are allowed for connection?