You are looking at the wrong list. Shadow Panther rate the Claw of the Netherwing Flight at 250 just like the result in the Off Hand list. It is rated 581 in the Main Hand list wich is the other formula.
( 2.9-1.8 )*10 + 4.25*( 83+156 )/2 + 15 + 1.15*15 + .5*30 + 15 = 581.1
Thanks. I'm an idiot. I failed to use copy correctly with the Main Hand formula selected, and was thus using the same formula twice.