Open Ports when scanned externally

[#1]

Curious as to why when I scan my external ip address with nmap (externally to my home network) all the ports come back as open? Whats the reasoning for this? 

I know they aren't all open as I run a pfsense firewall instead of the plusnet supplied modem.

[#2]

Try running Shields Up from Gibson Research (grc.com) from inside your network and see what they say.

[#3]

This is just a glorified nmap scan so won't make any difference? 

I have scanned from multiple external internet connections and the result is the same - can anyone from plusnet clarify why this is? 

[#4]

How exactly is your network configured? Are you using any kit provided by us? If you're not, then this behaviour is solely down to your local network configuration.

We don't block any ports out on our network unless you have our server side firewall enabled.

[#5]

I am using a BT Openreach modem (white box) provided by plusnet that feeds in to a dedicated pfsense router. All ports closed bar a select few. 

As far as i know not using the plus net firewall