…because the shape of the water bottle isn’t uniform either. Part of my new year’s resolution is to drink 4 liters of water per day, so I spend a lot of time looking at those lines. Edit: 4 liters is a lot, but appropriate for my size and activity level. Without conscious consideration I don’t drink enough water. It looks blue because the bottle is blue.
As someone with ADHD, I often don’t realize I’m thirsty until I have a headache and am about to pass out. Having an even arbitrary water goal forces me to do the math every once in a while and go “oh, I should probably drink something today”