Stall speeds - Bottom of green arc

[mshac]

The green arc bottoms out at 80 mph in a 1975 337G, and at 65 knots in a 1978 P337H, which translates to 75 mph.

Question - considering the P337H has a higher gross weight than the 337G, how is its stall speed 5 mph slower?

I've heard the wing airfoil design changed in 1978, but can't confirm this.

I thought this would be a good thread to start out the weekend! Fly safe everybody.