And yeah, that usage of African-American has always struck me as a bit odd... Like all black people are somehow from Africa and all Africans are black people.
Shorthand for sub-saharan Africans.
However, compare the term "Asian" then go look up the conceptual region known as "Asia" and you'll see it includes many places we wouldn't normally call the people "Asians". It's the same with "African". "African" and "Asian" as racial definitions don't line up 100% with the geographical definitions: they're the same
word but they have different meanings in relation to different areas of study, different contexts. That's just how language works. Different fields of study develop their own internal rules for what the terminology means. So demographers and anthropologists developed their own definitions of "Asian" and "African" which are internally-consistent, and geographers developed
their own definitions which are internally consistent, but don't have any real connection to how the terms are used in other fields. e.g. when geographers argue about whether a particular country should be defined as being in Asia or not, they don't go and talk to anthropologists to ensure that their definition lines up with the definitions of racial groupings, which also uses the
word "Asian". Same words, completely different semantics.