Yes, it is very odd indeed.
Pretty much the same exercises are done for the lower body on LBY as on PS legs floorwork, if you stand back and look at the video with a wider perspective. In PS legs, however, we do the exercises with a body bar or dumbell placed on the leg to increase resistance, while we lie on the floor pretty much doing leg raises for the inner and outer thigh muscles. However, the same exercise done with just the stability ball in LBP is much tougher and I had difficulty last night squeezing out all the reps. Why should this be? I can only assume that we are working the same muscles from ever-so-slightly a different angle, and the human body being the finely-tuned machine that it is, this slight difference of angle is all that's needed to render the exercise challenging again.
The exercises on the ball for glutes and innner thigh are not so difficult and I find the inner thigh ones particularly effective.
However, I experimented last night with the glutes and the hamstring exercises with my leg/s resting on both stability ball, on a chair of similar height and on the step and the results are:
--for glutes, I got more effective burn with feet on the step or on the chair secured against the wall to stop it being pushed away from me.
--hamstring exercises with ball seem pretty tough because your body is exerting so much effort to simply keep the ball there and maintain your balance, so I wonder if it is actually possible to be working the hamstring on the 'non-resting' leg effectively at all?
I went back and did this series of exercises twice for the hamstrings, once with chair and once with foot on step another time, just to make sure I actually felt the burn in the muscle and was working it properly.
Conclusion:
you are going to have to experiment and see what works for you, depending on your likes, dislikes and particular bodily strengths. There are some exercises for which I will spurn the use of the ball because it does not work the muscle group effectively for me.
As with technology improvements in other aspects of our lives (for me undergraduate instruction), 'new' is not always 'better' and can be just downright 'gimmicky'.
Clare