i think it depends on the gun you're getting shot with to determine how many shots. head shots definitely kill faster than anywhere else, so it does take that into consideration. might have been invincibility glitches in the videos. i've read that's been going on fairly regularly, although i have yet to experience it.
haven't delved too deep into the stats, but i think ranks are determined by your kills to deaths ratio. that's the most important stat i think.