That reminds me of a post I found on Reddit once. It was a screencap of a Reddit comment thread. The OP of the comment thread was basically saying this the whole time:
"Here's my experience about being anorexic and fat. I gained weight even though I didn't eat a single thing for several years. Yes, I know about the law of thermodynamics. Yes, I know about the law of conservation of mass. Bodies do not follow those laws, because of that one time I swear I didn't eat a single thing but still gained 100lbs. Your body sees the lack of food and tells it to continue storing fat. What do you mean, fat can't come from lack of food? It happened to me! I swear!"
In reality, they may have been eating without really realizing it. I know I do that sometimes. I'll go in the kitchen for a glass of water and leave with a bag of chips that I've already started munching away on just because the bag had been sitting next to the glasses...