This question, "Discrete Units of a Continuous Quantity", asks whether units of a continuous quantity should be spoken of as discrete or continuous.
The top answer states
The rule is simple, and you obviously know it: discrete quantities require the use of "fewer" and continuous quantities require the use of "less".
Calorie, as a unit, requires the use of "fewer". Energy, as a continuum, requires the use of "less".
"Fewer calories means less energy."
I do not understand the logic here though. Calorie is a unit, yes, but it is a continuous unit (it is possible to have half a calorie, or 4.582394 calories). Since a calorie is such a small unit of energy compared to what it is usually used for (food energy and human metabolism), it is very seldom expressed in a fractional form and instead is rounded (usually to the nearest 10 or higher).
Why then should "fewer" be used with calorie instead of "less"? Is there another relevant rule than just "continuous quantities use "less" and discrete quantities use "fewer"?