Your cab driver is using a variation of the mathematical consequence of "large numbers". Here are a few variations of the "trick":

1. A guy goes around to various towns in a drought stricken area and claims he can make it rain by beating on his drum. He even tells the town that they only have to pay if it actually rains. The guy then beats his drum. If it doesn't rain he leaves. If it does rain, he gets paid.

2. A change lady at the slot machines shows you the machine that will hit the jackpot. You don't even have to tip her if doesn't hit. Most of the time she doesn't get a tip, but occassionally it does hit.

3. The fortune teller gives you a money back guarantee that she can predict the sex of your next baby. She thus gets to keep 50% of the money that people give her.

I could go on about the cab drivers that predict football winners, but you get the idea.