My eating habits are awful. Why don’t I gain weight?

I’m a short female in my early 20s. If I had to guess, I probably eat 2500 calories a day and I don’t exercise regularly. But I consistently stay at a BMI of about 21-22 and my body fat percentage is about 23-24%. Am I just lucky? Is my metabolism just good because I’m young? Or am I way overestimating my caloric intake?

Also, I go through periods of eating less (because I’m busy or stressed or whatever) and I don’t lose more than a pound or two. During periods of eating really poorly, I don’t gain much either.

This is not a humble brag and I really hope it doesn’t come off that way, I just don’t understand. I see posts on social media about women my height trying to eat only 1200 calories a day and I just don’t get it.

Am I going to outgrow my ability to eat whatever soon? Do I need to change my habits?