Is it generally accepted that gifted children struggle athletically?! Because DD doesn't smile She's advanced in swimming particularly, and was an early bike-rider, monkey-barrer, cartwheeler. Also very good at gymnastics and ballet. I think because she has exceptional listening comprehension, she understands quicker than anyone what she's supposed to do, and her great memory means she can follow all advice at once (rather than say swimming with straight arms but forgetting to kick, or whatever). Plus she loves learning new stuff, whether it's a science experiment or a dance routine or the rules of a team sport.
My guess is that gifted babies are quicker to see the potential benefits of rolling/sitting/crawling, are more observant, better at mimicking, keener to learn to skills, and better at remembering the do's and don'ts from earlier physical attempts smile
Perhaps the better question should be not why are they physically advanced as babies, but rather why do they stop (if that's not just a misconception)?