I might be on the wrong track, but I thought the Mazda6 GX was a more "agressive" body than the Mazdaspeed 6. In our opinion the Mazdaspeed6 is easier, more neutral, and hence provides a more stable platform (more stability) than the more "agressive" GX. So I would rate the Mazdaspeed6 as a more stable body than the GX, and I also think the GX has more steering, due to the higher downforce, than the Mazdaspeed6.
Maybe I'm reading the chart in the wrong way?