That isn't how that works.Please go wikipedia to find out more about how computer generate random numbers.
No. It does GUARANTEE, in computer or programming language. Which you refer as, the actual game files.
Let's say the Rod of Discord. It is a 1/500 drop chance from Chaos Elementals.
Every time you kill one, it checks a random number from 1 to 500. If it gets a pre-determined one (probably either 1 or 500), then it drops the Rod.
It does not progressively get easier as you kill more, though that could make an interesting mod with less grinding.