Armin’s voice could always be heard at the start of each episode of Shingeki no Kyojin. It would show that he was the one who was narrating all that. Perhaps this proves that everything that had happened, is nothing more that him reminiscing about his past, retelling it to someone. Maybe he already has grown up, that’s why his voice sounded more mature when he was speaking at that time.
Maybe everything that was happening… has already happened.