I can think of two reasons. I didn’t experience any of them, so I’m just guessing.
The opponent let a 6th player join after the draw of that particular weekend. That player cannot play since the draw has been made, but he can support the teams.
or
One of the managers of the other game in the tournament joined your game. He can support the side of his choice and watch the game. Just like a regular game between two players in your league.