In my opinion, this Facebook post sums up editing:
In its early days [early 1980s?], the Freelance Editors’ Association of Canada sent its members a series of sentences to edit, to see which were the most common approaches to fixing some kinds of problems. We were in the very very early days of thinking about standards. One sentence, memorably, was edited by 101 editors. Only one pair of editors made the same corrections to it. So there were literally 100 different edits trying to fix a two-line sentence. And almost all of those edits worked perfectly well.
–Greg Ioannou, Editors Association of Earth (Facebook group), posted 16 June 2018
Every editor approaches a sentence in their own way, and applies the conventions and styles THEY are familiar with or have been asked to use. There are no rules — only traditions**, conventions, and guidelines. This is why I’m conflicted about editing exams and tests — whose ‘rules’ are you meant to apply? And whose ‘rules’ do the examiners follow in marking you? What is ‘correct’?
** Some of those ‘traditions’ and beliefs may have been embedded into your brain by your Grade 5 teacher several decades ago, and who’s to say they knew what they were talking about? Who’s to say they weren’t repeating what they’d learned at school several decades before too? How much was ‘assumed wisdom’, passed along from one generation to the next without question — or evidence?