I get frustrated with things like this sometimes too, but I have to remind myself that, in spite of the formulas used, there is still some randomness involved.
If you flip a coin, there's a 50% chance of heads and a 50% chance of tails. But if you flip it 100 times, even though the odds are equal every time, you could end up with 65 heads and 35 tails. It's just how it works.