So, is there some inside joke about the two Mexico's?

There's a Lower (Baja) Mexico, and a Southern Mexico ... did neither one of them want to be Upper/Northern Mexico?

Or did someone who doesn't speak spanish name them? It would have made more sense if the northern one was named "Alto Mexico", and then the other one either stayed "Mexico" or became "Baja Mexico". It makes no sense for the northern one to be "Baja Mexico". Basically, unless there's an inside joke, the wrong one is named "Baja".

So, I'm kind of curious to hear the explanation.

Baja Mexico is more of a reference to Baja California which flooded due to global warming. In the years following the earthquake of 2066, much of Southern California (and much of Northern Mexico) has been repopulated by people who have taken to calling the land Baja Mexico.

While it may not be geographically accurate, it fits conceptually for the time being.

We have plans for that region, which may end up becoming something else entirely.