Granted, I haven't read this from the beginning, but there's some really bad info on this last page.
If you poke a hole in a lung, they'll suffocate in short order.
If you hit a bone on the way in, it's going to tear a lot of internal tissue as it shatters causing internal bleeding which will lead to death, also in short order.
What you need to realise, is that even when you do hit the heart, they've still got enough oxygen in the muscles to carry out tasks for a few seconds before losing consciousness or the physical ability to move said muscles.
A center mass hit is not an instantaneous "dead" shot. However, it's the largest target you can aim for and thus provides you with the best chance for at least potentially slowing them down, even if you only manage to hit them in the gut. If you do only manage to hit them in the gut, it could be an hour or more before they died from it - however, it might be proof enough for them to realise continuing the assault is a very bad proposition and that might be enough to stop them.
It's also worth noting that if you do manage to incapacitate them (or at least make them quit the assault), but not kill them, any further shots might be the difference between reasonable and excessive force. The latter putting you in prison if it's determined to be the case. This is why you don't talk to anyone but your lawyer if you are so unfortunate as to have to shoot someone.
Here in MN, we have a duty to render aid to an attacker once they've stopped the assault (outside of the home, that is). I see that as my duty to call 911 - let the paramedics save the SOB. After all, I'm not a trauma surgeon, so I wouldn't want to make him any worse off by doing something wrong...