i used to think i was because i couldnt gain weight well but i was just bad at eating in a surplus. i think, now, that they are someone who's metabolism adapts very quickly. ie, i can eat 500 over maintenance and in a month my maintenance will have gone up by like 200 cals. i waas gaining on 2400 cals last year and now im stuffing myself with almost 4000 cals just to get some size. at 6' 160 my maintenance doesnt seem like it should be 3500