Hardcover book: 28x22cm image doesn't fit on the 28x22cm cover

Hello everyone!

My laptop is:

MacBook Pro (Retina, 13-inch, Early 2015)

Processor 3.1 GHz Intel Core i7

Memory 16 GB 1867 MHz DDR3

Graphics Intel Iris Graphics 6100 1536 MB

System: OS X Yosemite 10.10.4

Problem: I'm creating a 28 x 22 cm hardcover book. I've created an image with exactly the same size, but it doesn't fit on the cover. How will it be when printed?