Sometimes pages are revisited to double-check when a particular tag has not been found. Audits are optimized to run faster by applying intelligence around which tags need to be re-verified when they are missing from a page. Instead of revisiting all pages where a tag is missing, the audit reverifies when a page is missing either the most commonly used analytics tags or missing any of your custom tags. If the tag in question is also found frequently on your site, the page will be reverified.
Pages that return 400-level codes are not automatically re-verified unless they are missing a commonly used tag that is found frequently on your site.