They are using different methods of calculating rate.
Here is a free online calculator:
http://www.auto-ware.com/calcs/swaybar.htm
The Hotchkis hollow bar is .156" wall, Hellwig hollow is .188" OD is 1.125 but the powder coat can add .020" which you don't want to count.
I recall Speedway rates with a flat 5deg twist which will be 1" with an arm length of 11 3/16" only, the other arm lengths listed are useless numbers. 1st gen Camaros have a very short arm. Some calculators use 1" arm movement on each arm, so your calculated rate is double. It's best to take the dimensions and re-calculate them using one formula or online calculator. The bent hollow bars are not Chrome Molly steel, the Hellwig solid is and so is the Speedway hollow. I think the Modulus of elasticity is different between the two but most online calculators have the modulus Pre-entered so you can't change it.