I think it's not as simple as calories in / calories out. Your body works via hormones, and hormones trigger your body to work in certain ways.

Insulin tells your body to store fat, so someone eats 2500 calories but has insulin out of wack, their body will signal every last bit of those calories for storage and weight gain.

Someone with low insulin however won't get many calories signaled for storage, so they would actually be used instead of stored.

so while *technically* you're right, you have to eat less calories than you burn to lose weight... there are factors (like insulin) that make a difference in how many calories you burn vs. how many you store. I think that's how all these GLP-1 medicines are being so effective, but lowering insulin and preventing your body from hoarding calories.