When you go to the multiplayer screen on the default launcher and the default installation, some servers that are obviously online show as offline. When I join these "offline" servers, I am able to join just like normal and nothing really changes. (The server can also just show "pinging" for a really long time, might be related).
Edit: This is pretty random from my experience, to recreate I would just open the installation of Minecraft, decent chance that it happens to at least one server.