How do online security checks work?

[muno]

I tried two online security scans.
scan.sygate.com and www.grc.com shieldsup.

Sygate scan reported that all the ports it scanned were stealthed, grc shieldsup reported that all other, except port 21 was stealthed.

I have a ftp server running at port 21, and it should be open, as grc noticed. Why did sygate claim the port to be stealthed when infact it wasn't?
-M

to be honest i dont like grc "scanner". i feel the website is towards the average computer user who is not as interested in secuirty as other people

if you really want a nice secuirty scan i suggest you download

http://www.gfi.com/lannetscan/

free 30 days, it will show you what you have open

[muno]

Yes I'm aware of that tool, but unfortunately I can't run it. Of course I can run it from my local network but that does not show the vulnerability of my lan from wan.

And yes, too, I've read the flamewars where people debate whether what gibson does is good or bad

I was merely wondering how come the different results, why did sygate tell me everything is stealth, when infact everything is not. Port21 should be open as gibson noticed.
-M

[muno]

HELLO!!!1!
Anyone have any insight?

[muno]

Now I'm running the server on port80.
As before, sygate reports it stealthed, and grc says it's open.

I'm beginning to think that scan.sygate.com notices I'm running their firewall, and just plain says everything's stealthed