I think I know what it is, and it is a server bug. When counting the number of users, the server excludes clients that, according to some old logic, would be kicked if a new client joined. One of the criteria for that was that a certain amount of network packet timeouts had occurred, and that happens frequently between rounds when there is heavy network traffic from object deletions and other stuff (which we probably should optimize away some time). Luckily, the timeouts are no longer a criterion for kicking a user out, otherwise the lower displayed number would mean that you can push your way into a full server 

 I'll fix this for 0.2.8.3.
Edit: to let you verify it, I also updated Fortress to that code, although it is mostly running the code targetted for 0.2.8.2. I didn't have the heart to force a restart, it already did one half an hour ago 

 So you'll have to wait four hours before you can test the new code.