Its a standard probability calculation. The chance of two events occurring is the probability of the first event x the probability of the second event (which is only correct if you can correctly determine the probability of both events and one event is not related to the other event). For example the chance of raining today is 30% but the chance of you getting wet is 20%. That doesn't mean you have a 6% chance of getting wet if it rains today, its probably much higher.

I don't want to get into the law of large numbers vs the the law of averages (not the right thread) but its an interesting topic. The law of large numbers makes the calculation correct.