The dpi setting merely calculates the size in pixels from the physical size and the requested DPI (dots pr. inch), so if it’s 2"x3" at 300 dpi, it’s 600x900 pixels, but the file written is still “tagged” as being 72dpi. I remember this being mentioned before many, many years back, and obviously it still hasn’t been changed. I think @nathanletwory wrote a little plugin for it, but it might only be for renderings, not view captures.
HTH, Jakob