You can not measure images with a ruler on your monitor. The same size image on a 15" monitor and a 21" monitor are not the same. Those dimmensions shown in that dialog box in Photoshop are based on the DPI number you set. Yours is set at 72dpi. If you uncheck the box there that says "Resample Image" and then change either the dpi setting or simply change the width to what you want it to print at, then Photoshop will print it at that size. It changes the dpi setting to whatever is needed to make it that size. By unchecking the Resample Image checkbox you are telling Photoshop to not do any changes to the actual pixels, only to the information it uses to scale that image for the printer. But be aware, 72 dpi is fine for a monitor but is quite low for a typical printer these days. You may need to render the image at a higher resolution in the first place in order to make it look OK on a printer.