It does cycle through leg-focused worlds, arm-focused worlds, and core-focused worlds. But it's true that it focuses on lower body first and foremost.
My brother who is a fitness trainer said that it's typical of workout programs to start with lower body first, as that not only strengthens your leg and butt muscles, but it helps you improve your form for when you start working on core and arms. So it seems there is a science to it. He also says that people typically ignore his recommendations and jump right to arms and chest and end up hurting themselves in the process... <_<
Though I will say there are less red (arm) workouts in the game than all the other types, and it's made even worse that some of them are simply "raise your arms in the air" types of workouts. I had to stick to some lower damage attacks just so I could squeeze some good arm workouts in until I unlocked some better ones.
When I worked out on my own, I always neglected my core, so I'm kinda glad I'm being forced to do core stuff, especially early >_> But that form stuff does make sense it seems like