I agree that doesn't make any sense especially considering the fact both Kaido and King stated he had CoC. Zoro should have realized he has and was using CoC. Unfortunately we can't do anything about that.
The most logical explanation is he mistook CoC for a stronger CoA. If you go back and look his battle against King he was in control of his power. When he hit King with bird dance we can see he coated his sword with CoC right before he attacked. It was just one sword.