Question about PC Flank.

I'm using NetVeda as my firewall and ran a couple of different test on it.
With PC Flank their advanced port scanner test shows my firewall as being stealthed one minute then a port is closed the next minute.
I ran this same test over a couple of more times with the same results.
I did the same with GRC's Sheilds Up but everytime it says that I am completely stealthed.
So what gives with this PC Flank testing?
Why am I stealthed on one test and the next test I have an open port?
Sorry but someone may have to explain this to me slowly, I'm still in the newbie stage with firewalls and this is confusing
I have an idea of how to set up my firewall, I'm not that new where I just run it with the default settings

 

You may find some answers in this thread

https://www.wilderssecurity.com/showthread.php?t=114765

 

Thanks for the help djg!!