I know people say its due to resistance, but by what formula?
Ohms law, suppose you have a ground lead with a 1A current flowing through it an a 1 ohm resistance.
Then at one end of the ground lead, the one at the supply, will be 0V and the other end will be 1V. Half way along that lead it will be 0.5V. So if you connect something else half way along that ground lead it will not be ground but 0.5V.
Therefore connect all current carrying ground leads together at one point. This is called a star configuration.