With a 4-team minimum, your going to have very very few winning cards per 100.
If EVERYBODY just played the minimum 4 teams, you would not average 7 winning cards per 100 cards.......then factor in all the people betting 5-10 teams and you can see that the bookie is not paying on many cards.
Again, depending on the payoffs, the bookie will hold more than 23% on the total take on these cards over the longhaul.