Electrode difference between MUSE S and MUSE 2?

I am wondering how the last two MUSE models compare to each other in terms of electrode hardware.
If I understood correctly, the MUSE 2 also had to electrodes behind the ears. Why were they dropped? What did they measure? and how does the removal affect the accuracy of the MUSE S?

I am trying to learn as much as possible and fully understand it from a scientific perspective