I'd hypothesize that the "bars" body type typically has less weight in the lower body and a center of gravity closer to the torso. That makes it easier to use the upper body in the ways that are needed on bars - e.g. lifting the hips above the head for a handstand, rotating the body quickly around the bar, using the strength of the upper body to propel other movements like releases.
I think you are on to something. My dd has a very strong upper body and a streamlined lower half, with shorter legs. She can really fly on those bars, and usually gets all of her bars skills for the next level before she has all floor and beam skills.
Her weight ratio especially makes front tumbling a challenge. She has always struggled with doing nice front walkovers and handsprings. It's lot of weight to get up and over.