I made a Scribus document of two pages with 20mm margins left and right. I exported to a PDF file and printed the PDF file. When printed single sided, both pages had left margin of 20mm and a right margin of 24mm. When printed double sided, page 1 had a left margin of 24mm and a right margin of 20mm while page 2 had a left margin of 20mm and a right margin of 24mm. I thought this was something to do with Scribus binding as the PDF export dialog asks you to specify right binding or left binding but further experiments show that a similar effect happens printing directly from Pages. So I'm not sure now why my document is not printing centred on the page which is what I want.