One can not judge a book by its cover.

One may be slim and sad and one may be obese and happy!

The answer is that Nature’s  Laws of Health do not change for individuals. It remains constant for any one and every living being in this world. It applies equally for all. It is a generalization in the comments of people that is in question.

1. We see examples of many people without examining factors that go to maintain or improve health and we do not examine fully well as to what all health rules they were actually following in their daily life. Even a person who is taking all kinds of food that is not ideally healthy, but is eating moderately and is chewing the food thoroughly well, and that they eat only when truly hungry. Regularly they take care to eat some raw vegetables and some fruits.. There are people who are not stressed and they stay relaxed most of the time. There are people who have ample exercise either in their work or as a willing routine. They take good rest periodically. Even if one is adapting some of these rules one can be healthy.
2. We say that ‘they enjoyed good health’ but we do not know what all the problems each of those individuals had. Many people in today’s world look normal even while they have had two stents put in their arteries, they have had a triple bi=pass surgery. We do not know if they have had their intestine cut off due to an ulcer. Many people are carrying on like ordinary healthy persons, but they are carrying chronic diseases for many years and taking tons of medicines.
3. One factor which we recognize is the inheritance of a good fund of vitality that a person can receive as their inheritance from their healthy parents. This comes to some people as a reward of their good Karma of the previous birth according to Indian scriptures
4. Even among us so many of us who are following a Natural and healthy diet, do so many things that can spoil our health greatly. One small example is not to resort to eating when we are angry or extremely jealous of someone and upset. These bad emotions can entirely spoil our digestion by disturbing the secretion of enzymes or digestive juices. One leads to another and we end up sick even when we eat healthy food.So all these factors are to be considered when we try to understand what is good for us.