you have to remember, there has to be a path to the brain, or the heart. with mains, its easy for the electricity to path through your heart,because if you're grounded with your feet for example, and you touch a live wire, the electricity will pass through your arm down to your feet,making it likely your heart will be in the path. if you make a cut on 2 fingers on your left hand,and insert a 5v 1a probe on one finger,and ground on the other, the only thing you'll do is burn your hand,as electricity travels on the path of least resistance. this ground is called floating,as it is not the same ground as mains ground is(since its totally isolated in the charger/device),and this ground only exists inside the charger/device, so electricity will not flow through your hand to your feet like mains did,it will flow from one finger on your left hand,to the other finger on your left hand,taking the path of least resistance.
Why are you assuming a floating ground would be attached to a finger? All the phone chargers I've seen aren't grounded.