Who do the Grasshopper & GH2 Require Different Pinion Gears when Upgrading to a 540 Motor?

Famously the Grasshopper and Grasshopper II require a Lunchbox  pinion gear when upgrading to a 380 Sport Tuned Motor. However, while the original Grasshopper uses this same pinion gear when upgrading to a 540 motor, the Grasshopper 2 requires the Frog's 18t pinion gear instead. Why? And is this related to last episode's issue where the grub/setscrew in a Lunchbox pinion gear comes loose a lot in a Grasshopper Two?