McCarthy is my favourite author, but I find that I have to approach his work (and the dialogue) in one of two ways. It might be a personality thing, but I find myself choosing between reading casually, letting it flow over me, having understanding enough of what's happening to enjoy the book. Or I can read and re-read sentences to make sure I comprehend the detail, every word, etc.

With his conversations, I can read through at speed with a decent but imperfect feel of who has said what. But I find myself wanting to get it exactly right and re-read counting the odd and even lines of dialogue to make sure I know who has said exactly what.

It's rarely critical because the conversation in many cases is such a minute* part of the story (Spanish sequences in the Border Trilogy, for example). Maybe he's counting that as sufficiently well written, but maybe he's not the sort to want to read it exactly right. Like I said, could be a personality/spectrum thing.

*I'm talking about the quick incidental conversations more than the extensive speeches, such as from Judge Holden, where the speaker is almost easily discernible and the content is certainly a key part of the experience.