PDF size different from InDesign document size after exporting.

I see that here as well.

Background: page dimensions are stored in a PDF in points, and rounded to 2 decimal places, which is enough to cause the discrepancy when converting back to cm. I guess it depends on how Acrobat was designed to do the unit conversion. The file is still always saved in points regardless, so the difference means nothing.

As an aside, in Postscript, the dimensions are stored in points to 4 decimal places.

Interestingly, if you change your Units in Acrobat to Millimetres, it shows as exactly 1200 x 1600.

Looks like some kind of rounding error. What if you change the units to inches or picas?