step side rear ends wider?

[1moretoy]

I'm a little confused on the width of the rear end in my truck. Everything I have read states there were two different width rear ends for our trucks. 55.5" and 57" flange to flange. Mine measures 59". My truck is a 70 1/2 ton short step. Did step sides come with wider rear ends? I have converted to a fleet and I'm going to have it rebuilt, new posi, ford style axles ect. Either way I'm going to narrow it. Just wondering why it measures different then what everyone else says.

Thanks in advanced