Pedantic comment: There is no such thing as an Apple iWatch; they finally did it right and named a product without `i` in front. Just Apple Watch. My guess is the level of detail the "signals" can be seen is what matters. Apple Watch is good for some things but its not constant its only checking in from time to time to get a reading. Purpose built device probably has higher details (faster polling or something) than a small thing on your wrist.