Availability of the calories varies substantially, and day-to-day water retained varies on the order of a few lbs easily.
Just consider how much flatulence varies meal-to-meal, that's largely produced by calories guests living in your gut "burned" instead of you.
The trouble is it's a complex ecosystem hosting many lives, controlled by hormonal knobs affecting behavior/activity-level and energy storage, and the makeup of the food hugely affects how much is absorbed and at what rate.
Everyone knows firsthand when over-eating a massive meal of hard to digest foods there's a substantial amount of energy deposited in the toilet... And wild pigs / pet dogs don't get excited about eating human feces for lack of caloric value.
Just consider how much flatulence varies meal-to-meal, that's largely produced by calories guests living in your gut "burned" instead of you.
The trouble is it's a complex ecosystem hosting many lives, controlled by hormonal knobs affecting behavior/activity-level and energy storage, and the makeup of the food hugely affects how much is absorbed and at what rate.
Everyone knows firsthand when over-eating a massive meal of hard to digest foods there's a substantial amount of energy deposited in the toilet... And wild pigs / pet dogs don't get excited about eating human feces for lack of caloric value.
It's just an upper bound, nothing more.