If i remember correctly, that's some kind of Steam mix-up but a workaround exists:
a) Start your own server from your PC for a couple of minutes (just host a mission and slap a password on so people don't accidentally join). Apparently this sort of forces a "refresh" to the whole system.
b) Exit your hosting session.
c) Go back to the client menu and the servers should now list correctly.
I haven't had to do this yet because i never faced the problem up till now, but as far as i can remember that was the workaround proposed by everyone else.
Hope it helps
