Why the replacement fertility rate in human depends the country?

most demographic scientists believed for human the replacement fertility rate is about 2.1 with (because the sex ratio at birth is 105 boys per 100 girls and 0.05 children another to fullfill the early mortality. But in fact according to CIA world factbook this figure is about 2.07 in Iceland to 3.5 in Sub-Sahara Africa. What is the main reason