I think a third major thing is that people often have very skewed ideas of how many calories things have, like you decide to have a salad instead of a burger at a restaurant because it's the "healthy" choice, but often, because of the dressing, the salad will have as many calories as the burger, and then you feel like you were "good" so you have dessert!

Or people will try to do something like cutting the 30 calories of cream from their coffee, while leaving their 1,400 calorie lunch untouched, trading big expenditures in effort and habit changing for tiny calorie improvements.