I can catch every typo and fix the grammar so the page is clean. But actually editing the thing, seeing why a whole section drags or where the story loses its grip, is a different skill I don't have yet. I hand it back tidy and correct, and it's still not any better as a piece of writing.
More people experience this than they realize.
You can proofread, but truly editing a work is still beyond you.
If this sounds familiar, the Library can help you find the bigger picture.