Arbitrary X aren't Y can mean:

* All X aren't Y
* Almost all X arent Y (real numbers, computable)
* Many X aren't Y, some are, and the two groups are hard to disentangle (SAT problems, sovable in polynomial time)
* Few X aren't Y, but we haven't found a way to distinguish the ones who are from the ones who aren't, and can generate arbitrary X that aren't Y (violations of strategy-freeness in voting theory)
* Almost no X are Y, but same situation as above (don't know example)