Your pitching rate won't really have any effect on your FG. While some variance from the calculated FG is normal, yours appears to be well outside what I'd expect. A few things could cause this. First a very low mashing temperature would make the wort from the mash much more fermentable than expected. If you did a partial boil and didn't mix the top up water very thoroughly (harder than you'd think), you may have added too much, thinking your OG was much higher than it actually was. Also there's the possibility that an infection is causing the overattenuation. Hopefully it's not that, but I can't help but notice that it looks like you're taking your gravity readings directly in the bucket, rather than drawing a test sample. Are you certain you're following strict sanitation procedures throughout your process?