Looking inside an “A4” PDF produced by Dorico, the actual “page size” is specified in points, as required by the PDF standard, and is rounded to the nearest point, i.e. 595 x 842.
That actually corresponds to 297.04 x 209.90 mm (approximately).
There are 72 points per inch exactly, but since there are 2.834645669…etc points per mm, the “page size” in a PDF can’t be defined as an exact number of mm anyway.