If you go to the extreme, say you sample 100M people of 100M and 100M of 8B then the first number is exact and the second has a very small error. So the error i

If you go to the extreme, say you sample 100M people of 100M and 100M of 8B then the first number is exact and the second has a very small error. So the error is a function of sample size and population. In not extreme cases when your population is much bigger than the sample size you're correct that it doesn't really make any difference.