That is not a discrepancy at all. Genetics play a part -- it is clearly both nature and nurture. Some people are clearly predisposed to certain body types.
I was just disproving your statement that "its not about eating habits" where you were arguing it is just "nature"
That said "some people can and some people can't" -- well, that can be nature or nurture as well. I had a friend who swore she "couldn't" lose weight despite exercising constantly eating like a bird and could not lose weight. Everyone was like "poor girl has a hormone problem" It was a great story, but I got to know her, have dinners with her and understand her habits (my coffee = her double mocha, sitting on the sofa every night watching TV, dessert with lunch and every meal). She said she "can't" when she was just choosing not to. Put her in a room for a month with <2000 calories per day, she'd lose a ton of weight. If you burn more calories than you consume, you lose weight. Its physics, law of conservation of energy.