Theoretically, you can never have a 100% chance of an item dropping, unless you impliment that factor of running it an infinite amount of times. But that is physically impossible, and even then, if you run it an infinite amount of times, theres the CHANCE of it never dropping.
You have a .01 chance of it dropping lets say.
if you run it again, you haave a .01 chance of .01 to get the item out of that run, which doesnt improve your chances.
So running the instance 1,000 times, you have basically have a .02 chance of it dropping.
Its sort of like you can never get to a location if you go half the distance, then half of that distance, then half of that distance, then half of that.
Like lets say you are 4 feet from a desk
You walk 2 feet;
Then 1 foot
Then .5
Then .25
Then .125
Then .0625
Then .03125
And so on.
It goes 2+1+.5+.25+.125+.0625+.03125 = 3.96875
You can get very close to the almost perfect chance of getting it, but there is always a chance that in those runs that it will never drop.
And there is also the small fraction, almost not one, that you will get one every single time you run SFK.
percetanges for seperate resets where it resets the percentage, is just too hard for some people to grasp.