In making a new document by default Photoshop color mode, RGB resolution is 72 pixels / inch. For beginners design may not quite understand about tsb resolution RGB color mode. because with a resolution of 72 pixels / inch will result in less maximal image to print / offset by LPI 150/175. To print in general standard Photoshop file is resolution of 300 pixels / inch, not per cm for 100% usage of the file.
Comparison of resolution 72 dpi to 300dpi