It seems as though the calculations are something that Garmin is making, and not I.ICU. The “work” makes sense, as it follows the common convention of (avg W x 3.6 x hrs = kJs of” work”metric. My guess is that there are some data dropouts that result in a very minor difference between what Garmin calculates vs. what I.ICU calculates, as the I.ICU number is consistently lower by a handful of calories.
However, this leaves me more confused about how Garmin is calculating the kcal, which can be meaningfully different than the kJs calculated. I.e. ~418kcal in my recent 4.5 hour ride….