Comment 1 for bug 484616

The lack of pixel density information (usually DPI) is something that is quite noticeable in graphics manipulation software such as Inkscape and the GIMP, but also in other software that uses the DPI value to determine the size of the inserted graphic. Most software assumes 75 DPI if no value is set, making the scanned graphic four times wider and higher than the original when scanned at 300 DPI and inserted in a document.

Setting the pixel density value to the one used by the scanner seems like a very, and perhaps only, sensible default.