I think it arises at the point where you did not even consider the alternative. This is a very subjective thing, of course.
If the probability of the actual outcome was really negligible (with a perfect evaluation by the prediction-maker), this should not influence the evaluation of predictions in a significant way. If the probability was significant, it is likely that the prediction-maker considered it. If not, count it as false.
I think it arises at the point where you did not even consider the alternative. This is a very subjective thing, of course.
If the probability of the actual outcome was really negligible (with a perfect evaluation by the prediction-maker), this should not influence the evaluation of predictions in a significant way. If the probability was significant, it is likely that the prediction-maker considered it. If not, count it as false.