Why is the replacement birth rate 2.1?

Have you ever wondered why the replacement birth rate is usually quoted as being 2.1 and not 2.0?
Logically, you would think that if a couple has two children in their lifetimes, then the next generation would exactly replace the old one. However, according to the World Bank, for various cultural reasons, about 107 boys are born for every 100 girls on average in the developed world, and unfortunately (or perhaps fortunately) boys do not have babies. So, the replacement fertility rate is 2 × 107 ÷ 100, which is approximately 2.1.
A least that is the case in the developed world. In some developing countries this threshold can actually be as high as 3.4, and the replacement fertility rate globally is thought to be in the region of 2.33 children. Many, if not most, developing countries - including the European Union, North America, Russia, Brazil, China, Japan, South Korea and many others - are in a position of sub-replacement fertility (i.e. decreasing population, not including immigration and increased longevity effects).