This is an interesting observation, and something that scientist Tim Noakes brings up in an interview he did on the FastTalk podcast. Although I don’t tend to agree with many things Noakes says, he makes an often overlooked point about the “loss” of TDEE calories that he thinks should be considered for endurance athletes with long training sessions.
For example, ponder that an athlete has a TDEE of 2,400 calories with no exercise. This is the amount the body requires just to maintain weight and homeostasis with work, walking, RMR, etc. Then add a 4 hour training ride to the equation. Logically, one would think to take whatever mechanical work was produced in kilojoules and adding this to 2,400 to get the total energy expenditure for the day. For ease of addition, the athlete does 4 hours at 166 watts which equates to 600 calories per hour, times 4 hours, giving a total expenditure of 2,400 calories on the bike. Add this to the TDEE, and the calorie allotment for the day should be 4,800.
However, this is where Noakes makes an interesting point. Since the training session was so long, the athlete may actually “lose” or “miss out” on some of the TDEE calories that would be burnt normally during this time. While biking for 4 hours, no other activity is performed. No steps are being taken, no gardening is getting done, etc. If the athlete normally takes 15,000 steps during the day, spending 4 hours on a bike may result in less walking because less time is available. Likewise, while a person may normally do 2 hours of gardening on weekends, taking part in long training sessions may “eat in” to gardening time, and less calories are burned through that activity.
In summary, long bouts of exercise may burn a lot of calories, but the time spent exercising may reduce the amount of time left for other NEAT activities during the day. Thus, TDEE may have to be adjusted, and simply adding the kilojoules from your ride to your “normal” TDEE may lead to an overestimation of calories burned during that day. I have no clue if this is what the Garmin device is accounting for, but maybe?
MASSIVE DISCLAIMER - this is just a reference to what Tim Noakes said on a podcast. Personally, I find that if I put my weight and “activity level” into some TDEE calculator online and then add my kilojoules from riding to get my total daily expenditure, I feel like crap the next day. I’m 63 kg (139 lbs), I take 15-20k steps per day, and I ride between 15-18 hours a week. On any given day, I find that to maintain my weight and capability to perform during workouts, I eat anywhere between 400-800 calories MORE than whatever my TDEE + ride kJ’s ends up being. Example - my TDEE in any calculator is roughly 2,250-2,400 calories. Add 2,000 calories for 2.5 hours of riding at 220 watts, and I “should” eat 4,400 calories at most. If I do that consistently, I feel like a soaking wet sack of potatoes. Normally, on such days, which are most days, I eat at least 5,000 calories to feel full and happy. I don’t gain weight, and my training is productive. Just wanted to make that disclaimer to prevent anyone from just taking Noakes’ thoughts as gospel and eating less because some calculator told them to. Eat when you’re hungry, people.