Imho it's because you worked before asking the LLM for input, thus you already have information and an opinion about what the code should look like. You can recognize good suggestions and quickly discard bad ones.

It's like reading, for better learning and understanding, it is advised that you think and question the text before reading it, and then again after just skimming it.

Whereas if you ask first for the answer, you are less prepared for the topic, is harder to form a different opinion.

It's my perception.

Its also because they are only as good as they are with their given skills. If you tell them "code <advandced project> and make no x and y mistakes" they will still make those mistakes. But if you say "perform a code review and look specifically for x and y", then it may have some notion of what to do. That's my experience with using it for both writing and reviewing the same code in different passes.