Why is it harder to edit through something that's already written vs. Writing something totally new?

Sort of a question I have been asking myself today. In the world of technical writing on the side of matching a specific style, etc. I find it harder to edit an existing document vs. Just creating a new one and phrasing everything myself.

For my job, we have very strict standards/ word usage and whenever I have to edit a document from even 3 or 4 months ago a lot of things aren't standard anymore but I miss them because I have a mindset of this should already be correct.

Any tips for proofreading as a technical writer? / or books and videos about proofreading for standards.