Final drive ratio.
to get the 19:1 crawl ratio, the FDII has been geared down. So say the CVT was an actual transmission with gears 1-6. The final drive makes it more like gears .25-4 compared to a FDI. (I know the CVT has no gears, I don't need that lecture again. I'm just trying to give an example) So while the top gears in the transmission are basically the same, (between the 6 speed auto and the cvt) the FDII has that one difference in the final drive.
In my old FDII patriot, at 65mph, I would run at about 2800-3000 rpm, in a FDI you would probably bet at close to 2000 rpm. That's a huge difference in fuel savings.