It might also be a story of survival. There are stories of people getting lost in the wilderness for weeks and still making it out alive. Their will to live and the resourcefulness they showed in such a dire situation are just unbelievable based on a real story.
Another example is the story of Malala Yousafzai. She defied the Taliban in Pakistan to fight for girls' education. Despite being shot in the head, she survived and continued her fight. Her story, based on real events, is truly unbelievable in terms of her courage and perseverance.
Yes. 'Unbelievable' is based on true stories, which gives it a raw and authentic feel. The creators have carefully researched and adapted real events to bring this story to life on the screen.