When a physical event occurs, such as a medical diagnosis, we're always so sure of the fact, as if the condition is permanent. Yet even the most persistent conditions are often cured. They turn out to be impermanent after all.
We give so much credibility to what our eyes can see, but the world is actually unpredictable and unstable, not as concrete as we imagine. Change is the essence of physical reality. Change may take awhile, but rest assured, what seems permanent now shall be radically different in a matter of time.
On the other hand, with messages from God or prophetic dreams, we tend to double, triple, and quadruple check, then we still doubt. We like to give "logical" explanations for spiritual experiences, reasoning that they must be due to some physical event such as eating too much or not sleeping enough. Even if these explanations have no basis in fact, they seem more plausible than a spiritual experience.
Yet in retrospect the signs and premonitions turn out to be right after all, even after physical reality betrays us.