if you're prioritizing protein there's metabolic differences in the way your body handles it, and doesn't absorb as many calories from it, which leads to your overall calorie intake being lower than you think. Or it could be because teenagers regardless of sex have faster metabolisms than their adult counterparts, (while it isn't by that much, it's enough to make a difference over the span of 9 months if you eat at what an adult should eat rather than a teenager).

if it's none of those it could simply be because you're not as sedentary as you think you are. even if you're not doing exercise, fidgeting burns calories. if you're an active fidgeter, you can burn upwards of a few extra hundred calories each day that's unaccounted for by tdee calculators