Qt's printing system does not query CUPS for printer options until after the program opens a QPrintDialog and the user presses the Properties button to create a QPageSetupWidget. In particular, this results in an incorrect choice of A4 paper when the system is defaulted to letter paper.
Somewhere down the stack (QPdfBaseEngine? I was looking through the code but wasn't sure where this code should go), CUPS options should be queried sooner so that a QPrinter on its own without a QPrintDialog would pick the settings for the default printer and switching printers in QPrintDialog should switch the settings to the defaults for that printer.
I imagine the latter could be accomplished by creating the QPrintProperties dialog immediately, but I imagine Qt would like to avoid allocating that when unnecessary.