I do agree with you on people's reported caloric intake. Most people inaccurately record their eating habits. I use an app to get an idea of what my day was like.
If you put 100g of broccoli in a bomb calorimeter and burn it in pure oxygen, you heat it up by 34 kilocalories.
If you put it in a human digestive tract instead , the human can absorb anywhere from 0 to 34 kilocalories of food energy from it. We already have ample evidence that the food energy absorbed may be affected by the microbiome in the intestines, the preparation method used, the time of day, the time of year, and other foods that may have been eaten at around the same time.
But the calorimetry does establish a hard upper limit on absorbable food energy.
This is why non-ruminant herbivores with shorter digestive tracts--like the arctic hare--will engage in coprophagy. They will eat their "first-through" turds, because there is still some food energy in them. Ruminants do essentially the same thing with multiple stomachs and regurgitation.
If you aren't weighing your food and burning your poop in a calorimeter, you are inaccurately reporting your caloric intake. Even then, you can't really know for sure how much of that energy was absorbed for use by human cells, and how much was used to sustain your microbiome.
No matter what conditions you may have (or think you have) the above method will tell you exactly how much your total daily energy expenditure is, including what passes through you that you don't absorb, so you can know how much you need to eat to lose, maintain or gain.
I did say all the thing i posted there was for one meal. 10 Pcs of KFC Chicken, may be you should look it up? I bet it is over 3000 calories. And this was for JUST one meal! Likely Dinner, because lunch I would have rice, or Kebab with Chips. The Coke i drink. Which was 4L / Week. And Ice Cream.
Why did i remember all these so well? You may ask. Because at one point in life I was counting every penny because i spend too much on food. I was running out of money and I was desperate. I can't eat enough. I was constantly hungry. And Starving make me depress. I used to joke about How i eat all these and walked back to College in the cold weather i would burn off 1000 calories instantly.
Like some have suggested below, I guess it has to do with A) BMR, I think living in Cold weather make things a little different, and B) Not everyone absorb every Calorie they take! And that was my point. Some people simply don't absorb very well at all.
Sometime after the age of 24, something changed, I started absorbing, Within 2 years my weight went from barely 150Lb to 200Lb. It took me 2 years to slowly adjust my appetite. To eat less, and now days i am counting every calories I can to slowly shake off the weight. It is very hard, just as hard as I was trying to gain some weight many years ago.
Edit: And the 3 - 4 Pound Beef, Now i am older I know a single pieces of US prime Stripling is around 15Lb. My mum bought me over to my aunt's house and have dinner, They have half pieces of Prime Striplion left, that is about 7Lb. It was my first time ever to have US Prime beef and it taste unbelievably great! The dinner was self served and I was eating like there is no tomorrow. I had half of that pieces and was continually eating, my aunt found out and had to pull that striplion away from me to leave some for others. My mum then stare at me saying how i embarrassed her. At the time i thought I was hungry, why can't i eat more....... My mum later had to buy back a present to my Aunt as good gesture.
Not true at all, you weren't even counting your calories you're just throwing out a number. And if you were counting you were counting wrong. That's a huge part of people who say they can't gain or lose weight. They claim to eat these vast amount (or extremely low amount) of calories when they really have no idea. People trying to lose weight will look at a burger and say "that's about 400 calories" and someone trying to gain weight will look at the same burger and say "that's about 1500 calories", when in reality its somewhere around 900. Do that for 3-4 meals a day and boom, you've got your "I eat 4000+ calories and can't gain weight".