Comment by n3storm
True, any page oriented software like LibreOffice, Inkscape, Gimp, will show you US Letter sizes and US Letter Envelope sizes and you may have messed up with printing on wrong size... but as other posters say, maybe this days nobody prints on real paper anymore...
They all default to ISO sizes for me.
If I format the page size, Libreoffice does offer "Letter" and "Legal". GIMP shows them as "US Letter" and "US Legal" but again they're not the default.
It wouldn't surprise me if most non-US users hadn't seen them at all, and certainly not that they don't realise the US uses a different size.