When branches of a fault tree are pruned, their probabilities are not fully transferred to the “all other” branch. We test 3 explanations for this underestimation of the “all other” probability: availability, ambiguity, and credibility. In an experiment, we varied the credibility of a cover story and separately observed the generation of a fault's causes to isolate availability, and the categorization of causes to assess ambiguity. The results identify biased availability as a broad threat to the validity of likelihood estimates. Ambiguity adds to the problem whenever tree designers are unable to eliminate it from causes or categories. Finally, though subjects had clear expectations for what constitutes a credible fault tree, none of the “all other” underestimation could be traced to credibility factors. The discussion covers both underlying mechanisms and corrective techniques.