If you join a game in progress, you wont receive any points for winning the match and for finishing the match. That make a sense, however it would be better to divide total amount of points reward for mentioned goals by number of total rounds in the match multiplied by rounds a player was played.
This issue is the most obvious in Clash mode. Mostly I join to game in progress, but when the battle is over most players disconnect and I have to find new server. I hope you see my point.