I'm sure they have some criteria in curating posts, it was almost impossible that they are just upvoting randomly. They might be following the un written rule in the steemit world about curating the posts that are worthy to be upvoted - Good Quality Contents

Also I am reading a lot of answers about this matter and it always have this manual curation - meaning they are reading our questions and answers manually. This will then be distinguished if it was an answer or a question worthy to be upvoted