There’s no real explanation; each pastry is different. No two pieces are identical. And the crust is softer than dough would be.
But that’s what makes it so good.
Some say it’s the same as brioche, which they also call pain perdu or lapin vert.
And just like brioche, it’s traditionally filled with a sweet jam (in some regions, it’s almond or raspberry).
There are many types of pain perdu, including a pain éclaté, which is made with a green or red jam.