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.
Video is very different from printers.
And our goal is to figure out how this stuff works. The screen is typically larger than our photographs, that is, enlargement is often used to show them. We often scan at higher resolutions to fill the screen. When we increase scan resolution, it increases the image size. But a little goes a long way, and there's no advantage in wrestling with overly huge images just to discard most of the pixels when we display them.