Actually you don't burn a lot of calories standing up. YOu burn a lot of calories when you move.
Otherwise i agree with melancholy.
But i will put it in my own words too
The calories burned in BMR are the calories burned keeping you alive. Keeping your organs working - breathing, blood circulating, kidneys going, brain ticking over etc etc. This is your metabolism.
As said, The extra amount is for your activities. As you can see keeping your body alive takes a lot of calories. Some of this number will be because of your size and some of it will be pretty uniform between people of the same height and gender. But you've said your BMR is 2400. Are you sure that's correct. Your BMR and maintenance won't be the same. I suspect your BMR is lower because if i remember correctly, you are not a tall person.
You might not be reading the right part of the screen.
look here - though i am not sure if the numbers i put have stayed.
I put in
28 year old male
5 foot six inches
100kg (probably more than you are but i can't remember)
sedentary lifestyle
Ok now look carefully
This man's BMR is 2083
and his maintenance calories are 2500
To lose weight by 1kg per week, this person needs to eat only 1400 calories per day - which is interesting but i thought a man should not eat less than 1800. But perhaps that's becuase it is assumed that the average man is more like 5 foot 10. So if i were this man, i'd still go 1400 perhaps.
PS, i nearly misunderstood something important on your post so, can you please not use text speak on a forum. Please spell out words in full. It's standard etiquette on public forums. Thank you.