Believing in fiction
There is a difference between believing a story and believing in the story. One is accepting the story as literal truth, and the other is suspending your disbelief in it. One is a matter of accuracy, and the other veracity. Both are important, but in different ways and for different texts.
In the context of nonfiction, for example, you are writing with the intention of being considered a source of true information, whereas the goals of a fictional story lean more in the other direction: being internally believable but not actually believed in reality. However, in fiction we can sometimes trip up on a tension between believability and realism. And this is where the distinction really becomes important, because a story being unrealistic doesn't make it unbelievable (otherwise we wouldn't see so much success in the realm of speculative fiction).
Have I managed to confuse you yet? Let's break it down.