I i think you are misinterpreting maths and probability. I mean, its pretty much guaranteed if you look at it statistically.

You calculate odds by taking the odds of the thing not happening and take it to the power of your number of tries, then subtracting that result from 100%.

So if the case is 1/20000 (or 0,00005) chance of something happening, the case of it not happening is 19999/20000=0,99995. So if you do 20k kills the odds are 1 - (0.99995^20.000) =63.2% (basically 1/e) so its pretty high after 20k tries but far from guaranteed indeed. However! If you get 180k kills its a whole other story. Doing the math again: 1 - (0.99995^{180.000)=99.987%} of at least getting one wooden circle in 180000 kills. So yea it is guaranteed... i just debunked ya