I would guess it's because it requires more active stabilization of the back and core and as a result muscle tone and posture either stays the same or slightly

I would guess it's because it requires more active stabilization of the back and core and as a result muscle tone and posture either stays the same or slightly improves. I also find that I'm and moving around more often or changing positions more frequently since switching to an exercise ball. I'm guessing several short stints sitting are better than 4-8 hour sessions.