Drop rates are
chance
to get something, not a guarantee that you will get one after a certain number of kills/actions.
That chance is always exactly the same: so if the drop rate is 1/512, it is still 1/512 for every kill/action, irrespective of how many have gone before:
chance has no memory
.
~~~~ Just another victim of the ambient morality ~~~~
As others explained, it's chance - not set in stone.
Let's take hellhounds as an example. The "drop rate" for hard clues is 1/64. You can kill 64 and no clue. You can kill 128 and no clue. You can kill 300 and no clue (I've been there). However - you can also kill 12 and get 5 clues in those 12...which has also happened to me before. I think my record was 15 or 16 clues in like 5 minutes which isn't even 200 kills. As others said it's like rolling a die, though I think Jagex compared drops to being more similar to a wheel. Now if you just start on killing hellhounds it won't be 1/64. Might be 1/200, might be 1/5. The "drop rates" come from larger quantities. If you were to kill, say, 90,000 hellhounds it's almost a guarantee you'd have received around 1400 hard clues from them...give or take 10-20ish. When you do the math you come within the ballpark of 1/64. This applies to almost all RNG where there aren't prerequisites to receive the drop / things that effect the rate.
Casual legacy trash of 19 years in RS, author of 6 books irl