Nice. And yes networking can be fun to explore.
In general (very basic) if your pc initiate a call out the the internet, then when the reply comes back it is "ok'ed" by your pc because asked for it. And ports are opened. But if your test to see if it is open, well it can't reply or wont because it is in use (in this case) by dcs :).
Also a little tip. If you have a small power outage (blinking room light etc) and your internet experience is really weird, then it might be a good thing to restart your router. That 4 millisecond blink in power can be like an explosion to a router.