When displaying a diagram at zoom factor 1, the relation between space units and cm or inch depends on the screen resolution of the display. E.g. modern TFT displays typically use a display resolution of 96 DPI. (Older CRT displays typically used 72 DPI.) Thus in the case of 96 DPI and zoom factor 1, a diagram element with width or height of 96 will be 1 inch (or 2.54cm) in size on screen.
When printing at (user-defined) scaling of 1, 72 space units will be 1 inch (or 2.54cm) on the print out. This resolution is independent of the printer's actual DPI.
Please see what is scaling, and does it effect margins and created graph size for a more detailed explanation.