Example: You just posted the first chapter of a new story, and then someone instantly gave it 1 to 3 stars for no apparent reason. You check the grammar, and there's nothing seemingly wrong with it.
Ratings have nothing to do with grammar specifically since anyone can enjoy anything, and stars are a representation of someone's enjoyment of the story; it can have all the bad grammar in the world or a story as flat as Earth (I gotcha

), you will still find people enjoying it.
Of course, certain people will troll and put one star to stories just because they didn't "like" the story. Others will analyze and try to deconstruct what you put together to offer a rating representing (for them) your story.
Also, do not consider 3 stars a bad rating. Let's look at a scale:
- 1 Star should be considered bad since it is the minimal amount you can give.
- 5 Stars should, hence, be considered excellent.
- But what represents 3 Stars then? Middle, a story that is nor especially good nor especially bad. This can change between individuals but it is a scale that everyone could eventually agree on.
And finally, to respond to your original question:
I feel the same way as when somebody gives a 5 Stars rating or review to somebody with barely any written or published chapters on his story: confused. You can't judge a story based on some 5+ or 10+ chapters, especially if the story is tens or hundreds of chapters long.
Thank you for coming to my Ted Talk. I shall now return to my infinite slumber.