chances are they are measuring their caloric intake improperly.
unless they eat only canned or packaged food, or they have a food scale and they know the calories per unit of weight that they use, then their caloric estimates are hardly accurate.
and a person at around 150 pounds, around 5'10", should be eating 2300 calories a day at maintenance levels, meaning to gain weight, it is a requirement to shoot high above that.
if they are above 6 feet tall, eating 3000 calories a day would still be a slow way to gain weight.

protein intake and genetics are also important factors, in that the percentage of weight that you gain that is muscle is dependent on those factors, but if you eat more, you will gain weight.

Yeah, all of that plays a part. All I'm saying is that it is harder for some due to multiple reasons, such as not being able to eat as much and of course the metabolism thing. Everyone is able to gain weight but it is genuinely harder for other people. And you also mentioned genetics, which is pretty much what people are talking about when they are talking about "high metabolism". I also agree that the type of calories affects weight gain greatly.

I don't personally know the people I was talking about, but they are somewhat experts on this topic, and even made a book about it, so I doubt that they were estimating their diets incorrectly.

But what do I know? I'm on my way to gaining weight as well now, since starting to lift and overeating (healthy) food. At least I hope.

only 50%

just russian things bro