This metric ruler is imaged at 100 dpi and displays at aproximately the correct size. This metric ruler is imaged at 300 dpi and displays at aproximately three times the correct size.
Printers can print at various resolutions.
The 300 dpi metric ruler image if printed at 300 dpi will be approximately the true size. If it is printed at a higher resolution it will apear smaller than true size. Conversely, a lower resolution will produce a larger, less well defined image. This assumes the absence of any enhancement of the image in the printing software, e.g. autofit, autoscale, fit to page, etc.