I've wondered that many times myself.
Could be that the extra power lets them run a larger pump that puts out more pressure, or they can set the relief higher, and not worry about any added power draw because of the extra horsepower. Regardless, I'd love to know or certain.
I just wrote LS USA an email, hopefully they can explain the reason.
btw how are you liking your R4047H? any issues? I'm going to replace my old case sometime this year. My dealer is selling the G3038H for $17k, R3039H for $20k, and XR3037H for $21.5k. G3038H is good enough for me, but I kind of wonder if the R/XR series worth the extra premium?