There are also differences between gaining weight, getting fat, and being fat.
People gain weight when they gain muscle, and heavier people by the mechanics of supporting their weight seem to gain and hold muscle more easily than lighter people. More muscle mass has been associated with better health outcomes despite other factors.
I've also read that it's not being fat that's bad for you, but getting fat. Some people are unable to shed subcutaneous fat they gained at one point in their lives and carry a stigma of being fat despite being otherwise very healthy, even in their diet and lifestyle. It's a common idea that not shedding that kind of fat is a problem, but it probably isn't as much as people believe.
Being apparently not-fat could also be an indicator of worse health if you generated corporeal fat more easily than subcutaneous. That's what's being referred to when they say "belly fat" and apple bodies being associated with health problems, not the love handles and soft flab. Many people who look healthy because they're not fat could be skinny-fat, have higher corporeal fat, and not take care to exercise or watch what they eat, thinking that their weight and apparent size are indications that they're doing everything right.
But in cases of obesity, I do see people cheating their diets, focusing on easier but less important factors like steps per day, celebrating their daily water fluctuations as weight loss, and otherwise coming up with endless tricks and reasons to justify not not-eating.
There are many people I've never seen eating, but their trashcans and purses always fill up with food wrappers. They don't count that 900 Calorie Starbucks because it's just a coffee, and Weight Watchers says that's 0 points. Or they didn't use all their points last Tuesday, so they can have a treat today, and probably tomorrow since that doghnut just made up for last week and doesn't count.
That's certainly a broken regulation system, but it's one within their capacity to change.