apple watch workout: pool walking

i use an apple watch series 7 to record workouts. after recording my swim i usually pool walk, which is not, but should be, among the preloaded workouts. when i record the pool walk as "other," the calories burned is twice the number than if i record it as an "outdoor walk," even though my vitals, effort, and heart rate, etc. are exactly the same. why? what exactly is being measured that the designation should result in such a large difference? my arms swing more slowly walking in water than a regular walk, but so what? i thought heart rate and other vitals were how such determinations were made. apple should be able to improve its algorithm to properly record this popular exercise. people have been asking about this since the watch came out.