There is a bug with the server browser. There's actually a lot more players than the server list lets on. The problem occurs when somebody tries to manually join a server through the console, and then for some reason drops or disconnects during this process. Once a player does this, the server is 'removed' from the master server list and goes as unrecognized.
I'm not sure how long this 'lasts' for, or what the fix is. I'm not a server admin. Although I'm pretty sure the servers don't disappear permanently from the server list. More information is probably available on the Epic Games UT3 Forum.