They may be the type who don't vote unless they really like the story, and then they only give it a top score.
Disclaimer: I'm only a reader, not writer. I've been a participant in forums, and at other times "read only". I've seen far too may participants "flamed" for an innocent comment. Like "this is the way to build it, and any other idea is absolutely idiotic, and by the way you should be tortured".
I pretty much only give top scores (SOL and Lit). If the author showed skill in holding my interest, or caused me to "feel". If the author also has grammer, spelling, or (what's the word for... flow of content and putting his/her ideas on paper?), then I may send a private comment on it AND offering encouragement and praise for what was good.
I also consider it unfair for me to score a story that hits on topics I personally do not like (willful cheating, torture, and such. (SOL has tags, Lit only puts stories in categories.)
I've seen low scores. I figure there are enough people who are letting the author know how bad it is. Maybe they are being flamed. Maybe the low score is a compassionate person hoping to share the truth gently.
At the end of a story, if I just don't care if I ever see the story again, then I don't score it. Maybe I should give it an average, but by the time I see the story listed, it already has a score - usually average or less. As I've aged, I've tried to live and let live, not be judgemental, live in peace, and encourage good in others.
I may be butting in where I should not, so feel free to advise me to stay out.