Unusual settings can contribute as well. In a series about nature, if a particular species is found to have a behavior that is completely different from what was previously known. Such as a deep - sea creature that has a unique way of hunting or reproducing that is not seen in other organisms. This novelty makes the story unbelievable in the sense that it challenges our existing knowledge.
The true story often involves real-life events or inspirations that the series is based on. But specific details can vary depending on the source material and creative interpretation.
One could be the story of a person who survived a near - impossible natural disaster. They were in the middle of a huge flood, and against all odds, managed to find a small floating object to hold on to until help arrived.