You're making this stuff up dude. Of course calories in vs calories out is the main driver behind weight gain/loss, and physical activity and what kind of nutrients you are eating also determine if that weight gain/loss is mostly muscle or fat.
But are you really pretending genetics don't play a part in it? Guys like Romero or Costa don't have the same genetics as guys like Askren, Anderson Silva or DC. Some dudes can gain weight and gain little fat and mostly muscle, some dudes have a harder time gaining weight at all, others have an easy time gaining weight, but find it hard to stay lean. Regardless of the somatotype labels (which might be just an arbitrary, yet useful, convention) there's obvious vast individual differences, even with people in the exact same training schedule and diet.