It could be due to their personalities. For example, if one is extremely outgoing and the other is very shy. Their different temperaments would create an unlikely combination for a love story. In such a case, they would have to overcome their differences to be together.
Unlikely friends can get along through mutual understanding. For example, if one is a predator - like a wolf - and the other is a prey - like a sheep - they need to understand that they are different from the norm of their species. The wolf might refrain from its hunting instincts and the sheep might trust that the wolf won't harm it. This could start from a situation where they both face a common threat. Like a forest fire that forces them to rely on each other for survival.
It could be their social standing. If they were well - respected in the community, like a church leader or a successful businessperson, people would not suspect them easily. They were probably seen as having too much to lose to engage in such a heinous act. Also, if they had a family and a good reputation, others would find it hard to believe they could be a murderer. Their daily life might have been so ordinary and positive - looking on the surface that no one would think they were capable of taking someone's life.
An unlikely way to tell an unlikely story might involve having multiple timelines that intersect in unexpected ways. Or maybe setting the story in a completely unique and unfamiliar setting that challenges the reader's expectations.
I'm not sure specifically as I haven't read it. It could be about a girl who is in an unexpected situation or has some unusual qualities or experiences.
A solid plot structure. An unlikely story often has a unique or unexpected plot. But if it's well - structured, with a clear beginning, middle, and end, and the events are linked logically (even if they're strange), it can be made believable. For instance, in a time - travel story, as long as the rules of time - travel are set and followed, it can work.
Well, it could be about two parrots from different backgrounds or species that fall in love. Maybe one is a wild parrot and the other is a domesticated one. They meet by chance, perhaps in a park where the domesticated parrot is taken for a walk. At first, they might be curious about each other. The wild parrot is free and has a different way of life, while the domesticated one is used to human care. But as they interact more, they start to develop feelings for each other. They might share food, preen each other's feathers and start to form a bond that is unexpected to those who see them.