Inkscape is an SVG app, and does things "SVG-style".
One of the interesting things is that SVG follows CSS size rules in that
things are based on display units that may change. By default things are set
to use an assumed 90 DPI value. The difference between 72 DPI and 90 DPI gives
you the 4/5ths factor you're seeing.
In general, try multiplying or dividing by either 80 or 1.25.
This is a bit of a tricky area, as I believe that once we change things
to follow the specs even closer, it would entail getting the DPI from the
runtime display environment (though if not done properly, things would quickly
devolve into chaos)