Myne has never received any training in magic before.
Even books about magic are forbidden for commoners.
The only training she received was how to charge the charlices.
There was no answer, even later in the series, or even in the Q&A with the author.
If I had to guess, it's her luck & the gods' whim.
All magic is derived from the gods, so the religious texts she's been studying all day every day effectively
are books about magic. It's just that the church has fallen so far off the radar of the nobility that there aren't many truly magically talented people who are also experts on scripture. It's not that Myne is special, it's that everyone else is not even trying. (
Except Ferdinand.)
Later it turns out that
Yurgenschmidt is actually in a huge crisis rooted in the fact that the ruling class either don't follow religion or don't know how to do so properly. The whole country is running on a variety of extremely old, extremely powerful magic tools that have to be properly maintained using practices that used to be taught as part of the religious curriculum, but because the nobles treat the church with disdain and only ever send off the castoffs and dregs of noble society there a truckload of extremely important institutional knowledge has been forgotten. And the recent civil war basically killed off what was left of it.
The gods are NOT happy at this state of things, but they are happy with Myne because she's actually studying scripture religiously like an apprentice blue priestess should, so they more readily answer her blessings.
From the gods' perspective, in fact, Myne is doing quite literally the bare minimum of what's expected, and everyone else is slacking off.
Another implied element is that nobody told her she couldnt. She views magic and the gods very differently from native people and that gives her more flexibility in what she can do with both.
That's a huge factor as well. Whatever Myne comes up with that's strange or novel, Ferdinand usually figures it out almost immediately and can do it, too. He just didn't think of it first since he grew up with the "normal" of his own world.
Ferdinand is also very careful about not showing off everything he does know or can do.
His understanding of scripture and even the past at this point is even more advanced than Myne's, but he's already resolved not to pursue the path it lays out for him, because at the end of it he might have to either become king himself or be eliminated. He could do the same kind of blessing Myne has done, but he's stunned along with everyone else that Myne is doing it at her age, since he gained the necessary knowledge at the Academy, which Myne won't attend for a while.