First of all, the nutrition facts given by USDA doesn't make sense to me. I'll refer only to skinless facts. Chicken breasts have 120kcal and chicken legs have the same. I know chicken breast have more protein and less fat, however I think chicken legs are much fattier.
When the are roasted, chicken legs have a higher caloric value (174 kcal) than breasts (165 kcal). The same applies for chicken thighs, which have very similar values.
I tried to calculate the amount of nutrients/calories in a chicken leg, but I got incongruent results.
I weighted full chicken headquarters without skin and it was 266g. I bake it in the oven at 400F/200C for 40 minutes and it reached and internal temperature of 200F/93C.
Cooked weight was 225g. Bones was 64g. Therefore:
Raw meat: 266-64 = 202g ; 202g * 120kcal/100g = 242 kcal
Cooked meat: 225-64 = 161g ; 161g * 174kcal/100g = 280 kcal
So 280 kcal / 202g raw meat = 139 kcal / 100g raw meat
Why didn't they match? I think the are very well done, so they should have lost a lot of water. But they would have to lose more water so the calories match (In particular meat would have to weight 139g cooked).
Best Answer
The numbers in the USDA are empirically derived. They take several pieces of raw chicken (or rather, find others who have done that) and measure the calories. Then they publish the average number of calories between these pieces. Separately, they find measurements for cooked chicken, and publish that average.
Empirical measurements are never exact. The chicken pieces vary in calorie content, the measurement procedure varies, the calibration of the instrument varies, etc. So, you can expect quite a bit of spread. And it seems that this is what you are seeing here. The measurements of raw and of cooked chicken don't match up - this can happen. You usually cannot say which factor was responsible.
This also is a good reminder that it doesn't make sense to calculate calories to the gram, since the piece of chicken you hold rarely has as many calories as given on the label or in a database. The exceptions tend to be highly processed foods which have little variation.