On Sunday, 12 February 2017 16:09:19 UTC, Roger Hayter wrote:
You fail to allow for the possibility that it is getting denser.
Slightly less unlikely than the size of A4 changing significantly.

Paper can vary significantly in density depending on how it is processed during manufacture, and thus change the bulk/caliper.

Book paper is often a looser, bulkier weave than bond/writing paper.

Owain