This is a common question that I see asked in writing groups, especially in ones regarding editing. Many analogies are shared, ranging from air traffic controllers to health inspectors. An error missed is a job done poorly. There’s a lot at stake.
Is that true? Well, it’s complicated. And it’s worth a slew of articles on its own.
So let’s keep it simple and to the point here. Editing is tricky. Every word is a potential error, and then how those words interact with each other is a potential error as well. This has an almost exponential quality to it; one word is a possible error, two words is two possible errors, plus another potential error based on whether or not those two words can interact with each other. This continues with each word until you have a sentence, and then the sentences themselves are potential errors.
In essence, editors are expected to assess every word, every sentence, every paragraph, root out the problems, fix them, and then ensure that they haven’t caused other problems elsewhere. If an editor catches 300 errors in a document with 60,000 words, what they’ve actually done is assess well over 500,000 potential errors and come back with 300 confirmed problems. That they may have missed 15 errors does not disparage their skill; it’s just statistics at work.
This is why it’s important to go through multiple rounds of editing if you can. Keep in mind that two rounds of editing that both catch 95% of errors end up catching 99.75% in total.