From Dark and Darker Wiki
Yeah unfortunately your math is wrong. The way statics works doesn't follow your thought process.
If the chance of obtaining a Gold Coin Chest is .1%, and you can hit a pile 3-10 times, each time you hit the pile, the probability of getting a Gold Coin Chest remains at .1%.
In layman's terms, every time you hit the pile, you're only going to get a chest .1% of the time. But the more you hit the pile, the more likely it is that you get a chest.
I must note: if a drop is .1%, and you do X to roll of the chance of obtaining the drop 1000 times, you are not guaranteed the drop.
(Just like if you flip a coin 10 times, you may only get heads twice, even though it's a 50% chance you get heads!)
This is called relative frequency, which in this case, is 20%
If you flipped a coin 1000 times, you would get heads close to 500 times, but it's not guaranteed.
Binomial distribution:
Variance=n⋅p⋅(1−p)
n is the number of pulls from the pile, which is 10.
p is the probability of getting a chest per pull, which is 0.1.
So, the variance of the number of chests obtained from 10 pulls is 0.9.