GRASS raster display inaccurat

[grasslandtom]

A GRASS raster displayed in QGIS changes it's position relative to other layers when the map is moved with the mouse. I see this while doing the following:
_ in QGIS: add a GRASS polygon vector layer (let's call it poly)
_ in GRASS: `v.to.rast poly use=cat out=poly_rast'
_ in QGIS: add GRASS raster layer poly_rast
_ now move the map around with the mouse and watch the coloured areas of raster cells move around within the polygons they were made of.

I have the same polygon in PostGIS. When I add this too, the two vectors do not move relative to each other. So seems to be the raster which is displayed inaccurately. In my case it shifts several meters within a Gauß-Krüger (Zone 4) projection. The raster cell-size is 2 m.