What do you mean by this? Where are you checking the page sizes? What does resolution (which is what ppi is) have to do with anything?
If you set a page size up in cm or mm, the PDF should be the exact size unless you add bleed.
And even in Austria or France where the metric system is longer used than in any other country of the world (as it was introduced together at the same time) and in other metric countries we measure resolution in pixel per INCH!
And if you use mm in InDeign, you get an accurate page size, but it is true that in Acrobat it shows sometimes comas as internal all DTP software calculate in DTP-PT, but it is not an error, it is as it is.
Yes, I'm with Willi on this, in the UK we always use PPI.
I'm with Bob, neither InDesign nor Acrobat have a page resolution. You can force images on the page down to a specified resolution, but that has nothing to do with the page object itself or its dimensions.
You could inadvertently set a custom page size that's different than the document size.