I'm guessing it's an interaction between your genetics and how you react to food. Just like how some people are more prone to diabetes or don't every really get fat and don't manage their diets consciously.
I think they were giving the example that given the same diet, one person might develop diabetes while another would maintain a healthy weight due to predisposing genetic factors.
But actually it does seem to be the case that the ability to put on fat is in some ways protective against diabetes. In other words, people that easily store calories as fat don't have the metabolic problems associated with diabetes, since their body is able to effectively store any extra calories they take in. Diabetes develops in individuals who are not effective at converting excess sugar in their blood stream into fat, so instead they suffer metabolic damage. That's not to say that they don't put on fat, but if you see someone who has an enormous amount of excess fat, they are likely to have a much higher threshold before they develop the disease.
