Working Out and Not Getting Enough Calories?

I don't understand the whole working out but not losing because of not getting enough calories thing. Can anyone explain that to me? How is it possible to burn 800-1000 calories per workout but still not be losing weight? I thought if calories burned are greater than calories consumed then weight would be lost but that's not happening for me. Any insight?

Disclaimer: I'm not an exercise physiologist nor nutritionist nor PT. But.....Here's my understanding:

Couple things come to mind.... 1) not getting in enough calories can send the body signals that "we are in starvation mode, hang on to those calories!" so that's why it's often important to keep your metabolism going by ingesting at least the bare minimum to keep your organs going and not trigger starvation mode, 2) net calories are not the same as total calories (e.g. we burn calories to keep organs functioning and life going, regardless of what activity we're doing so while some one can calculate they burned 100 calories per mile they ran at a 9 min/mile pace, their actual net calories burned were more like 60-70 per mile, because 30-40 of those calories would have been burned whether the person was running or sitting completely still on the couch.), and 3) calculations on workouts are notoriously imprecise, as tons of variables exist in individuals (RMR, BMR) and external variables (like treadmills & ellipticals overestimating calories burned to sell their machines - - because one would be more likely to purchase a machine that tells them what they want to hear vs what actually is - - and things like MyFitnessPal that have member-submitted information.

Well it doesn't happen in the lab where people's actual intake and actual calorie expenditure is measured accurately. It only happens in real life where people report that they exercise and eat healthy but "don't lose anything".

However, in these cases, when I push for more details, I almost always find one or more of the following is going on:

-they aren't tracking their food so they have no idea how many calories they are actually consuming (and when they do start tracking, they often find they are consuming 2x as many as they thought)

-they are grossly overestimating how many calories they burn in exercise (for all the reasons noted above -- in studies it's been found that humans naturally over-estimate calorie burn and under-estimate food consumption)

-they've just started the exercise program (or recently upped their intensity) so they are retaining more water than normal (for muscle repair) and can cause the scale not to move for a week or two at first. (But in this case, they are losing fat and the scale will move eventually.)

Another thing that happens to people as they get close to their goal weight is that they start building up muscle at the same time they lose the fat so they get smaller (go down dress sizes) but the number on the scale stays the same. So they aren't losing "weight" (mass) but they are losing fat and size. Which is what we think of when we think of losing weight. And what we want to have happen.

But that doesn't happen if you have a big calorie deficit because your body won't build muscle if you have a big calorie deficit. So it doesn't tend to happen early out but much later post-op.

And very rarely they've increased their formal exercise but have counter-balanced it with a decrease in informal activity. So they start walking at lunch time for exercise but start taking the elevator instead of the stairs when not "working out" usually because they are tired. That generally only happens with extremely low calorie consumption and/or some sort of vitamin deficiency.