I often question this. It is something I really don't understand.
Gads, up until a few years ago, I was doing 4000-6000 cals most days. Other days, I was doing four shots of vodka and nothing else. I'm real thin, generally muscular, great health. Great health (to repeat) and quite capable of 20 mile hikes with a 40-70 pound backpack (absolutely no f' joke), at a very fast pace.
I still don't grasp this.
Also, muscles take energy to maintain. As you lose muscle as you get older your burn rate will also go down because of that.
Finally, most people eat at set times even when they’re not hungry and snack outside of set times when they are. Needless to say this automatically results in overeating. Skipping meals when you’re not that hungry is great but harder to do when you live with other people.
Also, if you were actually doing 4000cal one day and ~300cal the next (4 shots of vodka), your average cal count would still be 2150 across two days, which is perfectly fine.
Alternatively, if you actually are doing 20 mile hikes very regularly with that sort of weight, you truly could just be burning that much extra in calories. Long distance running/hiking burns an immense amount of calories.