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?

I struggled with this apparent paradox for a while before I discovered Hans Rosling's succinct 2-minute explanation for it. I link the video below, but I understand that this effect arises because of the demographic profile of the population.
The number of births/deaths can intuitively be seen as
$$\text{Number of Births} = \text{Fertility Rate} \times \text{Population of Women of Childbearing Age}$$
$$\text{Number of Deaths} = \text{Mortality Rate} \times \text{Population of Old People}$$
The reason we did not see the number of deaths overtake number of births despite a low fertility rate was because the population of women of childbearing age were relatively higher than the population of old people. This was because the people then in their 70s and 80s were born in the 1940's, when the population of South Korea was much lower:

Accordingly, over time we would expect South Korea's demographic profile to change to have more and more older people, which will cause the second term in the death equation and hence the number of deaths to rise. And indeed that is what we see:

Link to Hans Rosling's video: Hans Rosling Explains Population Growth with Toilet Paper!.
Charts taken from South Korea Population (LIVE) and Age distribution in South Korea 2021.
