chat history limit in MUC
The "chat history limit" parameter used in a MUC is supposed to be used when asking the backlog, so you can get some fresh history instead of what your client has saved from your last connection.
It apparently doesn't work as expected. The delta between the parameter value and the number of "fresh" displayed messages can be caused by the fact that not all the messages sent in the history are "displayable", but I think that there's a issue beyond that.