How is it possible that praline pecans have less calories than regular pecans?

It's not an awful lot of calories in difference. Pecans, like all nuts, are highly caloric because they're high in fat. It just so happens that your praline pecans replace part of the pecans with sugar, which is less caloric than fats. So 28 gr of praline pecans have less cals than 28gr regular pecans.

Reminds me of the “low fat” diet craze. “We replaced all that fat with salt and sugar. Enjoy!”
- It's the same with the lo-carb fad. No more sugars and starches, here's a wad of salty fat!
Seconding this. 28g of sugar has 108 calories.

Which roughly just over half the amount of calories in 28 g of pecans (193 calories)

So simple math.

28g sugar + 28g pecans = 108 calories sugar + 193 calories Gives you 301 calories total

28g sugar times 2 = 216 calories 28g pecans times 2 = 386 calories

Sugar has significantly less calories per gram than pecans.

When you go by weight, you can really play with the calorie content by substituting calorie dense food (like fats, nuts, oils, avocado) that has a lot of calories per gram with foods that are the same weight but less calories per gram

Carbs like sugar have less calories per gram than fats, and salt has 0 calories, but it adds grams of weight

The pralines are substituting some of the high calorie weight of the pecans for lower calorie sugar and salt. So the weight is the same, but the total ingredients have less calories because of the non-pecan stuff used to top off the weight to get to 28g)
- Which is why you should never just look at calories.
  
  600 calories of candy bars and 600 calories of steak looks a lot different. But will weigh about the same. From memory, that will be about 2 and a half Snickers, and one 8 ounce steak.
  
  Trying to eat 600 calories of healthy green vegetables would see your jaw fall off before you finished chewing. About 6 heads of romaine lettuce to get you that far. Until you add dressing. Which, generally speaking, will have about 120 calories per ounce. A little more than a candy bar.

Fat is more calorie dense than sugar so the added sugar would lower the calories per gram I think

If I were to hazard a guess... and this is ONLY a guess...

28g of pecans is 100% pecans.
28g of praline pecans is pecans + other things.

So you aren't eating 28g of pecans, let's say you're eating 22g of pecans and 6g of other ingredients.

If those other ingredients are less calorie dense than the pecans themselves, then yes, it could have fewer calories.

28g of pecans = 196 calories.
196/28 = 7 calories per gram.
Sugar = 4 calories per gram.
Butter = 7 calories per gram.

Sugar only has about 4 calories per gram, so 28 grams would be 112 calories. So a blend of sugar and pecans has less calories per gram (somewhere between). Fat is very dense in calories per gram, and pecans are mostly fats.

Folks have commented on the caloric density of fat vs sugar but I did want to highlight that Calories aren't the only thing that matters - we're much worse at productively using sugars. Praline pecans certainly shouldn't be viewed as a strictly healthier option.

This article indicates that the USA FDA allows for up to a 20% margin of error for values required to be on the Nutrition Facts label. The article also describes multiple methods of measuring the calorific value of food, either by burning it in the not-so-TSA-friendly-named bomb calorimeter or through methods that have standardized the calorific value of each constituent nutrient.

It's also the case that not every foodstuff is perfectly identical to all other products. A banana is hardly going to be constituted exactly like another banana, and even the most basic measurement of mass will not match up to other bananas. Yet some sort of "standard" banana must be assumed in order to print the nutrition label.

As an aside, I do fondly remember making a bomb calorimeter in chemistry class using a polystyrene cup as the insulation. It worked remarkably well, IIRC, being within 10% of what we were given as the expected value. Obviously, real measurements would be far more controlled than what some college freshmen can manage, but the concept is sound, if only measuring what the food provides, not necessarily what the human digestive tract can extract.

As an aside to an aside, celery appears to not be a negative-calorie food, even after considering human digestion.

That's what I assumed was the thing. I understand that sugar has less calories per gram, but butter is also a big part of the mix. So I assumed it would be much closer to the calories of actual pecans. But even a 10% margin of error would probably allow for that I would assume.

To expand, pecans are about 6kcal/g sugar is 4kcal/g - since the serving size is 28g, not by volume, they have reduced the pecans per serving and added sugar per serving which is less calorie dense per gram.

fat

Check the serving size listed in Google against what's on the label there.

His post says the serving size is the same.

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Shoosh that’s 570 kcal per 100 grams! And Half of that is sugar-calories!

Regular pecans have about 700 kcal / 100 grams - no sugar.

You could do a test: eat 100g of those Praline Pecans one day and 100g of regular Pecans the next day. And write up how you feel for the hours following! It helped me realize how bad sugar was lol