As both the helpfile and code indicate, you have a CHANCE to get one of X benefits. Meaning you might get X, or Y, or Z, etc.

So let's say you get X.
But you are ALREADY RESISTANT to X.
Then you won't get X since it no longer stacks, as you are already resistant to X.

So if you WOULD get resist poison, but you are a dwarf (already resisting poison), then you won't get anything this time.

But if you WOULD get resist negative, and you are a dwarf (not resisting negative), then you will be resisting negative.