I don't understand. Why is it that...?!

I don't understand. Why is it that...?

Some people I know go to McDonalds at least once or twice a week. They drink a few cans of soda a day. They go out all the time. I don't mean outside, I mean to the school clubs. They vary from skinny to sort of brawny.Yet I never go "out", I go to McDonalds only once a MONTH, throw up when I drink more than half a cup of soda (so I don't drink soda), and only drink tons of water and a 3-4 cups of any kind of milk. I swim at least 3 hours, 3-4 days a week in the summer (mostly play swimming) and work out w/dumbells at least once a week. My body fat percentage is around 21 percent. I eat fruit in modest amounts. I only like potatoes as a vegetable. I love red meat. I don't like salt. But yet, I am not skinny (which I don't want to be), not fat (don't want to be either), and not scrawny or brawny. Why is this?

6 days ago
My chest is 36", my abs are 32", my legs are 20", my arms are 12".

Some people are just naturally built differently and have higher metabolisms. I'm sure you could get to the weight you wanna be at eventually. I would suggest doing more cardio workouts all year, and not just during summer. You could join a gym if it's too cold to workout outside in the winter, or do something at home. I usually ride an exercise bike every other day and on off days I take about a 45 minute walk if the weather is nice.