"Fertility rate" and "birth rate" are related, but used to mean different things.

Observe that the first thing wikipedia says in the entries for both topics is, "Not to be confused with" the other.

https://en.wikipedia.org/wiki/Total_fertility_rate https://en.wikipedia.org/wiki/Birth_rate

My thought was that maybe this was a difference between British English and American English, so I looked up the word "fertility" in the Cambridge Dictionary.

It turns out that while the dialects share roughly the same first definition ("(of animals and plants) the quality of being able to produce young or fruit:"), British English has a second definition: "(of land) the quality of producing a large number of good quality crops".

In that sense, I can see why they used the word. They sort of invented a new definition, "the quality of producing a large number of good quality humans."

https://dictionary.cambridge.org/us/dictionary/english/ferti...

i think the word "rate" makes it clear they are not talking about just "fertility".