Measurements in Photoshop do not match 100% print view size

I am using Photoshop CC. My problem is occurs when I create a new file. If I make it with 15cm width and 10cm height (i.e A6 size), when I view it in 100% (i.e print size) the result on my screen is smaller than the paper I'm using. When I compare my school ruler against the Photoshop ruler, units on the Photoshop ruler shows appear to be smaller.

I have not printed it yet, only made it in Photoshop, but when I choose to print it ( Ctrl + P ) it looks like it should be the right size.

Any idea?