After around 50 helmets and pairs of ears were cast for the Stunts, the molds were quite worn and damaged, so for the Hero casts, which took place after the Stunts, the molds were softer and had lost some details. That explains the missing teeth and ear bump, and the slightly different overall shape.
About the lenses, as the Hero helmets were made for close-up shots, i guess they thought the bubble style had a more SF feeling.