When I see the approximations needed to count the 7 billions humans actually living on earth, I smile at the 110 billions number.

The honest answer would be : we have no actual clue on how many people ever lived on earth, and we can only guesstimate how many are living on it right now.

Census only register people that let themselves be registered. It's probably mildly inaccurate in rich countries and terribly inaccurate in poor or chaotic countries.