Hi @olesenreck
What Melanie is trying to say is, pages have a span4 span8 distribution because the page builder content is built primarily for use in 1/4 1/2 1/1 columns, which works perfectly for an 8 column content area. If we were to make this 9 columns, it would not divide up equally, which would require a vast amount of extra css to make it work.
The shop page has a span3 span9 distribution because the page shows 3 products per row, and span9 can be divided into 3 span3’s, whereas if it was span8, it could not be equally divided into 3 columns.
If you want to change the widths, then open up archive-product.php and change all of the span9’s to span8 and all of the span3 to span4.
However please bare in mind that your shop page may suffer some layout issues.
– Kyle