Since make makers can submit subpenny order, for example, if a stock quoted at 10.01X10.2, he can buy at 10.0101 and sell at 10.1999. even if he is wrong, he can sell back to 10.01 or cover at 10.2, losing only 0.0001. that means, MM will never lose. As long as he can capture all the liquidity, he would eventually win all the money.