In medical school, many doctors learn the saying, “When you hear hoof beats, think horses, not zebras.” Most physicians are taught to focus on the likeliest possibilities when making a diagnosis, not the unusual ones.
However, it seems that some physicians oversimplify diagnosis so that _only_ the likeliest scenarios are considered. It's also an opportunity for subjective biases to influence an individual doctors definition of "likely."
E.g. Trans broken-arm syndrome