hide server variables

[realdreams]

Those server tracking sites can fetch server variables, hlsw can also get some variables without rcon. But what they can get seems to be different.
So how do they get those variables? How to set a variable to invisible (at least can't know it without actually joining the game)? I am sure some servers make their sv_steamgroup invisible but on my server it's not.
I have no idea how to set permissions for those server variables.

Thanks.