Stefan, thanks for replying.
Yes the option is selected. Clearing it makes no difference.

Incidentally, if I create a new Word document, the default (presumably in the normal.docm file) is set to A4 and if I try to print the document it shows A4 as the size. It is only when going into Printer Properties that it comes up with Letter 8.5" x 11" (as per OP). This seems to be the Default size and also what Standard is set to in Commonly Used Settings. If I repeat the process with an Excel document, the Standard size and default is set to A4.

I printed a word document with the default settings (US) and also as A4. Both came out exactly the same, so I assume that the documents are being printed correctly as A4.

Its not really a problem, but there seems to be some sort of issue in the Printer Properties option when dealing with Word Documents (Office 365). Don't know if the Printer Properties option is a Microsoft program that is part of Windows 10 or not.