On video monitors, scan resolution merely determines image size. But we are always greatly limited by our output device, and often cannot take advantage of maximum resolution. The images are huge, and our screens are simply not large enough.
Video monitors are relatively low resolution devices.
A 17 inch monitor screen might measure 12.5 inches horizontally. A 15 inch monitor at 800x600 might be 75 dpi. A 14 inch monitor at 640x480 might be about 65 dpi. However, while the concept of those calculated dpi numbers is correct, the numbers have little significance to scanning. Neither the video driver nor the video board has any concept of screen size in inches, and therefore dpi can have no meaning for the images either.