I have looked at about 200 of my blocks and about 90-100 have perforations that have shown perforation measurements that do not line up with book/Library & Archives Canada/Actual measurements...I try to use only mint blocks for accuracy.
Here is one example...Scott 1673 "Book Binding" stamp.
Book measurements - 13.0 x 13.3
Library & Archives Canada database - 13.0 x 13.5
Actual measurements - 13.1 x 13.5
Something to consider when collecting stamps.
Here is my block of 9 that measurements were taken from.