You learn a passage, review it until it feels solid, and then move on.
Then a few weeks pass. You try to recall it in a conversation, or during the quiz, and the passage you knew inside and out is now full of holes.
That's not because you didn't try hard enough, it's because most tools let you recognize the words instead of produce them. First letters, multiple choice, easy modes, and recognition feels like knowing right up until the moment you really need it. So you lose two things at once: the words, and your trust in your own sense of having learned them.
Recallable works the other way. Every lesson asks you to say the words out loud, from memory, right before you'd forget it. You don't have to manage the review schedule or guess whether it stuck. The words accumulate because you've been recalling them the whole way, and you can trust that you actually know them.
