How can the fertility rate be below 2 but the number of births is greater than deaths (South Korea)? I read an article published in 2020 which claimed that the South Korean population had declined "for the first time", the number of deaths (~300,000) outpacing births (~275,000). However, this would seem to be mathematically impossible because the birth rate has been below 2 in South Korea for decades (see chart below from World Bank). How is it even remotely possible?

Update: Based on one answer there is the idea that the increase in life expectancy could be an explanation. This seemed improbable to me originally, so I didn't consider it. But in any case, let's consider it:

So we see the life expectancy has increased from say 80 to about 82 apparently between 2010 to the present time, while the birth rate has been well below 2 that whole time. So, I guess I don't know the math of this. How do you compute birth rate vs life expectancy?

This is very similar to the question
Why does the US death rate not "match" life expectancy
In my answer there I simulated what would happen with the deathrate when a sudden change in the risk of death occurs. This gives a dip in deathrate that stabilizes later

The dip is because, the current distribution of the population is still based on the old risk of death. The population is not a representation of the current state of risk of death, but has a history with different risks of death (as it was in the past).
See also the development of the population pyramid. This is the case of the Netherlands, but you can make a similar graph for South Korea. The number of young people is decreasing (less births) but the population increases because people have stopped dying. The volume of the population increases by having more people of old age.

