And 2100 kcal per day is not safe or sustainable for almost anyone that exercises regularly.
I’m a woman with a relatively large frame (~65kg/180cm) who used to do 14 hours of hard cardio a week. At that time, my recommendation was 2250, the first time in my life it had exceeded 2k. For smaller women, the recommendation is sometimes much lower. My stepsister is about 45kg and 155cm tall and her calculated daily calorie burn is like 1300. My ex boyfriend’s mom was told not to go over 1200, which I thought was the lower limit for humans generally- things are different when you’re a short, post-menopausal woman.
All that is to say, it’s probably an average of 2100 calories, spread between people who need on average 1400-1800 calories and those who need 2000-2400
It’s honestly wild the difference in caloric requirements based on age and sex/gender (I don’t know how much is due to size/hormones, so I don’t know where trans people’s requirements would be) even before factoring in activity level, so it’s entirely reasonable not to realize the difference.