There are hundreds of those cases, the most common one is "el agua", and the reasoning is that "la agua" sounds ugly. It is just that.
Discussion
La acha (h is silent) sounds weird.
But also let’s take a moment to appreciate that a masculine axe would be equally weird!
You also win :)
Yeah, that's what I've been told. You win the prize for being the 1 in 100 native speakers who understand their own language :)
Let me explain it better. There are situations when if you stick to the rules your expression will sound ugly. Esperanto is proof of this. So, in English this is solved by changing the pronunciation of the words, for example "In the end" the word "the" sounds different than in other expression because the "In thE End" is problematic.
In Spanish we cannot do that. Spanish forces us to map one to one the sounds with the text so, or we accept that uglyness in our language or we have to change the word and use a different one.
Yes, I see. Indeed in english we have lots of tricks like that (inserting 'y' or 'j', there). But at root it's as you say, not 1-1 mapped, even for non-trick situations like the two schwa sounds in 'banana'.