I know that's what you're asserting, but why is it that it's not high metabolism? Has there been any known studies that show/prove this?
Get any skinny person who says they can't put on weight even if they try, and get them to eat 3000 calories a day. I can guarantee you they will put on weight 9 times out of 10. Like I said, lots of skinny people overestimate how much they eat and figure they eat a normal amount, so they blame it on a fast metabolism. Or they'll eat a lot of crap that doesn't have any nutritional value, not gain weight, and blame it on metabolism. If they actually counted how many calories they ate in a day regularly, I'm willing to bet you'd see around 1500 calories a day. It's like the skinny person version of "I'm not fat, I just have big bones". They've never actually put the serious effort to track and eat in, so they'll just keep blaming it on metabolism. Hell, even with a high metabolism you can gain weight.