Consider the following case: I want to print a chapter from a study script. In order to save paper I want to print 2 pages next to each other. I can go to Printer properties > Page > Page Layout and select "2 (2x1)" as pages per sheet layout and "Left to Right, Top to Bottom" as page order. The preview (see screenshot) is misleading as it seems that pages will be printed one below other (orientation of the "text lines" in the preview).

The actual printing result is still as expected (see image).

See also https://bugs.kde.org/show_bug.cgi?id=404150, where I report this problem for Okular (part of the KDE project), which uses QPrintDialog.