Some players after joining the map, they will quit. When they quit, their country will be automatically controlled by the computer (AI). But the slot is still full because their name is still on the game list. Game admin can kick those players, so that the slot will be empty. And I believe many games before day 7, if the slot is open, new players could join and play that country. To answer your first question, see Hastings's post above mine.
I understand now..
