I hope I know how the algorithm works. I am the one who suggested and designed it

What he states there is exactly what I said above. You can see results of <10% because an individual outcome can be less than 10% as long as it's in the middle 80% of the curve (if it's not, the battle is re-run as Piranha says). The original idea was not to create a guaranteed victory but rather to smooth out the standard deviation so that you didn't get extreme results like an 8v8 battle resulting in one side winning while losing 0 or 1 man. The side effect of implementing it created what I termed the 90% rule where once the sum of your chances to win exceeded 90% you couldn't lose.
So when you are observing battles what you want to look for is when you have an 8v8 type battle with identical strengths that the winner will always suffer at least 4 casualties. Otherwise the battle will be re-run until that happen.
KGB