This is the USDA definition of the difference between Calories and Kcal.

The “calorie” we refer to in food is actually kilocalorie. One 1 kilocalorie is the same as one 1 Calorie. A kilocalorie is the amount of heat required to raise the temperature of 1 kilogram of water one degree Celsius.

In the UK we tend to use calories when talking about food and exercise although we actually mean kcal so I’m not surprised if you are confused.

Your garmin refers to 550 calories in the same way that the Fast Diet says men can eat 600 calories and women can eat 500 calories on their fast days.

If you really used 1874 cals then you should have been 201 grams lighter after your ride. 1874(grams) divide 9.3(cals per gram) = 201 grams. Weigh yourself and your drink bottle before the ride. Then weigh yourself and whats left in the drink bottle after the ride. This is based on the assumption that your using mostly fat as your energy source because you have been fasting. If its glucose the weight loss should be even higher.