Digital BW, The Print

Technical question. I've got this 21step.tif file and I'm dying to
know why the steps are in 9% increments. Step wedges that I create by
hand (gradient, blur, posterize=21) give 5% per step. All this time I
was printing this image out and I thought I was getting 5% per step,
and I'm not.

I'm just trying to do my own profile for BO printing, since nobody has
offered one. Can anyone give me a reason to continue using 21step.tif?

-Pieris
digital darkroom blog: http://www.pmb.net/darkroom