Because paper has a thickness, once your pages are folded the outer leaves will be wider than the inner ones, as they have to travel around the folded thickness of the sig. Careful printers will compensate for this by “shingling” the pages during imposition, so that the gutter margin on the inner pages is less than on the outer ones. Thus, by making the gutter margin different on every page do they make it look exactly the same on every page. Here’s a definition from PrintingIndustry.com. I’m not so sure that what they say about perfect bound books is really true. After all before the back of the sig is ground off it was nevertheless a sig. It will of course be true of a book printed on a digital print engine, delivering 2-page “sigs”!

I suspect we don’t hear talk of this any more because like so much of the crafty element in our industry, it has been taken into the software driving the imposition.