It is some thing to do with chat,not some one delibritly leaving when you enter.This often happens to me.OMG are they leaving cause I entered?Possible but I do not think so.
Chat some times shows people there when they have actualy left,if you refresh page the chatroom sorts its self out.
But if the chats background could be a permanent background,lets says windows then this would make things easier for people as a lighter background would show more text colours.

.No complaints about chatroom from me?LOL.