Why do I get different results from Plug in SEO than other SEO checkers?
There are many different SEO checkers, each with their own approach and criteria. It's impossible to score perfectly on all of them! Changing something might fix it for checker A but then break it in checker B.
It's best to choose one and stick with it. We're not able to answer questions about how other SEO checkers work but we're very happy to explain why ours is giving the results it does and how it works.