As formerly one of the people who can eat anything and stay skinny I will explain how this happens.
First when I was in high school I worked on a farm in the summer, played football, basketball, and ran track during the school year. It was very hard manual work and when the summer was over I was still very active not only did I loose weight I also became very muscular.
After High School I went on to College full time and worked a full time job that required me to be on me feet most of the time. I was active enough that I didn't worry about loosing my muscular build or gaining weight and never hit the gym.
After college I started into a much easier job but by that time I was also married and had kids. My sons and I would shoot hoops, play baseball, and a bunch of other fun activities.
Now that my sons are grown up and I have a easy office job that requires almost no physical effort I find that I need to watch what I eat and make sure I get out and be active as I have gained about 15lbs and and my six pack is now a four pack.