I mostly rate books based on personal enjoyment but I try to be a little technical too. 1 and 5 stars are ratings I only give if I have some sort of bias, it's hard to be objective because I either found the book personally offensive or emotionally validating. I rate with my feelings anyway but in the middle of the scale I often manage to put some of them aside.

1 star - I actively hated it, and I wish I'd DNFed it or DNFed sooner (I rate books and write a review if I drop a book past the halfway point, because I think "this book was so bad I couldn't finish it" is still valuable information). (examples: Light From Uncommon Stars, The Midnight Library)

2 stars - I didn't like it but it wasn't necessarily bad. To me it was boring, forgettable. (examples: Bonds of Brass, She Who Became the Sun)

3 stars - I liked it but it was just okay. I may have a couple of major complaints or the book wasn't "for me" for whatever reason. (examples: Dune, They Both Die at the End)

4 stars - I liked it a lot and have no real complaints, might reread one day, but it lacked that little extra "something" that would make me love it and feel emotionally attached. (examples: The Blade Itself, Mort, 11.22.63)

5 stars - I loved it and really connected with it, it was the right book at the right time, I'll be talking about it for weeks and years to come and almost definitely reread it at least once. (examples: The Martian, Assassin's Apprentice, The Long Way to a Small Angry Planet, Nimona)

I usually write at least a small review to explain my feelings, so that if anyone is looking to ratings/reviews for guidance they can get a sense of whether or not they should take my rating seriously. While I agree that it sometimes feels unfair to rate a book low based on gut feeling alone (like you said, it's hardly the book's fault if it doesn't connect), it's also how I and many others read. A lot of books that for me were 1-3 star books are very highly rated on goodreads and some of my favourites have received bad reviews, so at least for me ratings are just not a good guide.