What is actually in the pile
Run a handful of crusher run through your fingers and you will feel everything from small rock down to something close to sand. That is not a sorting mistake, it is the point. The larger pieces carry the load and the fine material fills the space between them, so when a compactor runs over the pile the whole thing locks together instead of shifting around under weight.
Clean stone, the kind with the fines washed out, will never do that no matter how many passes it gets, because there is nothing small enough to fill the gaps. That is fine for a spot where water needs to drain straight through. It is the wrong material for a base that needs to hold still under a slab.
The job it does under a slab
Once the ground is stripped and graded, crusher run gets spread over the subgrade in layers and compacted as it goes up, not dumped in one thick lift. On clay ground, that layer of compacted stone is what keeps the slab from sitting straight on soil that swells and shifts with the weather. It also gives the pad somewhere for water to move through instead of pooling under the concrete.
It stays under the concrete. Once forms go up and the pour happens, crusher run is not something you see again unless the slab is ever cut open.
Why the grading matters, not just the size
Two piles of crusher run can look similar and behave differently, because it is not just the top particle size that matters, it is the spread of sizes underneath it. A well graded blend has enough of every size between the largest rock and the finest dust to fill its own gaps as it compacts. A poorly graded pile, missing a chunk of the middle sizes, leaves voids that a compactor cannot close no matter how many passes it makes.
That is one more reason the base under a slab is not a place to guess. What looks like a reasonable pile of stone from the driveway is not automatically the right material for what is going under a slab that has to sit still for decades.