I am wondering if this is due to a different type of reed thickness, or plating technique on the reeds. They state that they are both Phosphorous bronze plated but the HP are much brighter. If you are familiar with the Hohner Basso Melodica it has a diapram as well to try to provide constant pressure maybe the difference is in the elasticity of the bladder material? I will ask a few friends that are in with Suzuki and maybe they can tell us the reason.
Melodica-Me