At a weigh-in many years ago, I was told that I have to consume ~3400 calories a day just to maintain my weight-- so I was, at that point, burning at least 140 calories an hour just existing. So I've always just assumed that the calculators take the base calorie requirement into account when calculating burn during exercise. I also take the numbers that any calculator provides with the requisite grains of salt-- they're all just guesstimating, but I imagine doing a better job of it than I could guess. I have a handful of calculators I check, but fall back to the numbers provided by the Garmin, they tend to be the most conservative.