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We have a blog post (actually several of them that have the same SEO characteristics) that brings a fair amount of traffic to our site (relatively speaking), and according to Google Webmaster Tools it averages in the top 10 in SERPs for various terms. This page has no external links to it, and very few internal links pointing to it. When I run Moz's On-Page Grader for the various keywords it ranks for, the page get's an F on all of them. It was not optimized for any keyword, and it isn't our best content; it was a blog post written a few years ago and forgotten about and was never promoted in any way. The topic does happen to be about something that people search for frequently. According to the keyword difficulty tool, all of the keywords it ranks for have 40-45% difficulty. We have lots of other pages on our site that we have tried to optimize and that get A's and B's in On-Page Grader, that have both internal (from the home page and main menu) and external links pointing at them, etc, but they don't rank well at all. Keyword difficulty for these keywords is in the same range, from 37 - 53%. Why does this one page rank so well when the other pages don't? Additionally, we have been looking at a competitor who has a page that ranks #1 in universal results for numerous keywords according to SEMRush, yet the page gets an F On-page Grader for those keywords. The page has 3 links to it, all from the same domain and it has a very low domain and page authority. The Domain Authority of this page is 47 and the page authority is 33  according to Open Site Explorer (compared with our DA of 30, and PA of 1), and the social metrics are a bit higher than ours, but neither has a lot (they may have 15 likes to our 10).  Why does this page rank so well for them? How can we get our Page Authority higher? Thanks for any and all help.