The writing is always fluid and grammatically flawless. This carries much more weight with us than we believe. I know the illusion well from decades of grading college papers. Many of the highest quality students use English as a second language, and I know this, but an American well trained in writing, grammar, spelling always gives an impression of superiority. (Being well trained in writing, grammar, spelling etc is of course high merit, which is how the illusion forms - it is basically an illusion of global 'intelligence')