Yes, I think often naturally slim people still regulate what they eat - just instinctively. So they might live off crap - I have a friend who eats a big cooked breakfast, then crisps and chocs are all she eats the rest of the day. It's rubbish, but if you work out the calories it's actually well under a maintenance level. Thinner people are usually more active too as it's easier for them. And men have more natural musculature which means their body has to burn more fuel every day.
Of course they may be thin but anyone living off junk food can't be very healthy. And it only takes one physical problem, meaning they do less exercise than normal or get a hormonal imbalance, or emotional problem leading to them overeating on the junk food and losing that natural regulator, and they can end up with a weight problem too.