Mesuare in Photoshop doesn't exactly when view in print size 100%

I am using Photoshop CC. My problem is when i make a new file with 15 cm of width and 10 cm of height (i.e A6 size). When i view it in 100% (i.e print size) and it smaller than the paper which i have. It mean i hardly to design for the best accuracy. So when i compare my school ruler and PTS ruler, the PTS ruler is shorter.

I don't print it yet. Jut make it in photóhop but when i use print it in PTS ( Ctrl + P ) it look like fit the size?

Any idea?