The server browser doesn't scan for games on your LAN like the in game browser does. It downloads a
list of servers that it will try to query. This list gets updated frequently based on the heartbeats that it sends to dplogin.com's master server.
If your server's IP is always showing up in this list, then the heartbeats are fine.
Some routers will not do port forwarding inside the LAN. It will only forward to ports in your LAN if the connection has come through on the WAN port from your modem. Your computer sends a query to your public IP, but once it gets to your router, the router realizes that the destination is actually the IP it has on the WAN interface and will try to accept the connection itself without using any port forwarding rules that would normally be applied to all packets that come through the WAN interface.
If you had another server browser tool that supports Q2, you could try to query the server by manually adding the LAN IP that the server is listening on and you would probably see what everyone else is seeing.
It's kind of hard to explain, but hopefully you'll get the idea..