I agree with birds - I ate very healthily as a kid (thanks parents!), and was a typical skinny kid until I was 18, when I had a full time job and started drinking, eating bulk junk food etc. My somatype would have been "meso".
I think that the genetics part is overstated - people have a tendency to over and under estimate their activity and eating levels (barring any other medical conditions).
I used to put it down to things like metabolism, until I realised that those people who I thought had faster ones were actually just a lot more active than I, and/ or were doing a version of intermittent fasting, rather than absent mindedly snacking etc.
That said, the more lean body mass that you're carrying, the higher your basal metabolic rate will be, and therefore you can eat more without getting 'fat'.
Most of my family are rake thin tbh.