1 Watt = 1000 Joules of energy per second
1 Kjoule = 1000 Joules
1 Kcal ~= 4 Kj (4.184)
However, the average person is just under 25% efficient, so of the 1Kcal, about 25% is transferred to make the bike move. The rest is excess heat. Therefore, 1Kj measured by a Power Tap is equal to 1Kcal burned by your body. (4 x .25 = 1) It’s appx a 1:1 ratio.

Unless the Garmins actually measure power, then it's all just an estimate. Question- do the Garmins show different cal expenditures if you do 30 minutes at 20 mph on a flat into a 15 mph headwind vs with a 15 mph tailwind? Serious question; I've never used one of them (I've never even seen one...)