I've been watching anime since I was 3 or 4 years old. Because of this, some of my scores are simply how much I enjoyed them when I watched them and might not be how I would score them were I to re-watch them. I've yet to score anything below a 4 since I search around online before starting a new anime. I'm not sure how bad an anime would need to be to earn something lower.
When I start watching an anime I try my best to finish it even if it struggles to keep my interest. The only reason I drop an anime is if I find it very bad or if it starts to get too repetitive and even then I might give it another shot.