The short of this bug is that a hitbox on a body that can be revived remains where it shouldn't be. This prevents most projectiles from passing through to anything behind it, including grenades, rockets, and even tank shells. Thus, a player behind this hitbox "death shield" is impervious to some types of damage.
Perhaps this explains some oddities you've encountered when playing Battlefield 4, because this sure as hell explains quite a few things from my time playing.
However is it deliberate that you can stand three meters away and revive? That is even more odd.
Although my experience with defibrillators is somewhat limited IRL, I am pretty sure it requires direct contact.