The only thing I can think of is ....some brands DO NOT include the gallons per minute to operate the power steering in there gpm figures available for different things.. For instance... If a tractor brand says the machine has 15 gpm gross available, and requires 10 gpm to run the tractor, doing the math says there are 5 gpm for an attachment... But what they may not say is it also requires 3 gpm for the power steering... You do the math from here.. This is just a for instance set of figures, but you get the idea...