I created a drawing whilst learning to use Inkscape and didn't pay any attention to the units I used. When it came to saving the drawing as an image, I decided I wanted the the image to be of the whole page and that the image should be 1920x1200 pixels. Here is my drawing and what I believe are the relevant settings:
Inkscape says the viewbox is 508 by 317.5 in "user units" but I don't recognise the numbers 508 and 317.5. I can see that 508 = 1920*xs where xs is the x scaling factor (0.24565) but I didn't knowingly choose "508" or "0.24565" - where did they come from?