Maybe because there a degrees in how well someone can use the CoO? Just like with the CoA or any other ability for that matter? Even if you can speak, that doesn't mean you are good enough to become an actor. Even if you can write, that doesn't mean you are good enough to write books. Even if you can run, that doesn't mean you suddenly can run an entire marathon. Rayleigh, Shanks, Pekoms...they could all hit the bodies of Logia users like they were flesh and bones, other characters however could only do it partially. And like with all abilities there will always be people who somehow have gotten a natural talent for it, that however doesn't mean that the ability in itself is somehow different.
In fact Rayleigh even did mention it. He said when he explained Haki to Luffy that people on Skypiea called this Mantra, which is also what that child's ability was called by the Skypieans themselves. Also the doctor who was checking Cobi clearly said that it was Haki. Finally Rayleigh knew exactly when Luffy arrived on Shabaondy despite that there was quite a distance between them.
This is plainly CoO, but like with all abilities some people have a stronger affinity for it than other people, it's that simple.
