If you love almonds (and you should - they're an amazingly healthy snack!) you can feel good about grabbing a few extra next time you're enjoying them. It turns out that about 20% of the calories stored in almonds can't be metabolized when we eat them because of the physical structure of the cell walls.
The same is true for some other nuts, seeds, and vegetables.
The old school method for determining calories was pretty simple. Put a controlled amount of a food in a chamber, set it on fire, and measure how much heat it put out. After measuring thousands of foods all the data was put into tables. Over a hundred years later, and we still use these same tables today to determine total calories in a food.
So in the lab a handful of almonds might be able to supply around 421 calories. But your body burns food differently and you may only be able to use 336 of the calories in that same handful of nuts.
If you're a food or nutrition geek you can get more details on the study here.
You can also check out the full nutritional profile of almonds here.