Why does a nmap scan on a router/firewall report different open ports when the scan in run from inside the network

I am testing/setting up a Thompson TG585v7 router/firewall.

when I run a nmap scan against its public IP address I get different results depending on whether the computer running the scan is inside or outside the LAN.

e.g.

Results when running from a computer INSIDE the LAN

$ sudo nmap -O 210.86.xxx.xxx

Starting Nmap 6.40 ( http://nmap.org ) at 2015-02-22 10:55 NZDT
Host is up (0.0049s latency).
Not shown: 995 filtered ports
PORT     STATE SERVICE
21/tcp   open  ftp
23/tcp   open  telnet
80/tcp   open  http
443/tcp  open  https
1723/tcp open  pptp

Results when running from a computer OUTSIDE the LAN

michael@trusty-ssd:~$ sudo nmap -Pn 210.86.xxx.xxx

Starting Nmap 6.40 ( http://nmap.org ) at 2015-02-22 11:10 NZDT
Host is up (0.038s latency).
Not shown: 998 filtered ports
PORT     STATE SERVICE
80/tcp   open  http
1935/tcp open  rtmp

Is this happening because

• When scan is run from outside LAN firewall responds with rules from INPUT chain?


• When scan is run from inside LAN firewall responds with rules from OUTPUT chain?