PDF export sizes vary based on document dpi

[AndyQ]

If I create a Designer document at 60 x 60 mm width/height @ 300 dpi  (in document settings) and export a PDF, the PDF document size is reported as 60 x 60 mm. Dandy.

If I create a Designer document at 60 x 60 mm width/height @ 72 dpi and export a PDF, the PDF document size is reported as 60.3  x 60.3 mm. Not so dandy. 

also note: even if the Designer-exported PDF reports as 60 x 60 mm, if that PDF is opened with Illustrator it results in a 60.029 x 60.029 mm artboard. There's something fishy with the underlying maths here...