Calorie counts in nutrition data are generated by something like burning the food in a bomb calorimeter. Which is most definitely not what our bodies do. We have all kinds of different biochemical ways of metabolizing different types of foods.

So someone eating 1000 calories of day could be eating very healthy or they could be ingesting a whole bunch of garbage full of sweeteners. It doesn't seem like anyone really understands how it works but all those sweeteners are more likely to make you gain weight.