They probably didn’t care about the participants of the wars and only counted the wars since [very long time of peace].
It just turns out that the inhabitants of Azeroth are not very good at math and didn’t figure out how to count further than three until BfA and this scientific breakthrough allowed them to label BfA as the fourth war.
Now, you may ask: Why not label any of the previous wars the fourth war then? Well, simple: Because they were old news. Half the scholars already forgot about them and the rest of the conclave lacked the resources to come up with yet another number after four. They tried to get the research funded, but no dice.
Seriously, though, I think it’s just a meta thing that’s best not questioned too much.