The increase in aleatoric uncertainty observed in the halogen-agnostic scenario can be explained as follows. So far, we equated the aleatoric and epistemic uncertainty to the non-variance-related and variance-related components of total predictive uncertainty, respectively, under the assumption that the hypothesis space H was well-specified and the problem setup (X , Y, P, H) fixed. Without this assumption, non-variance-related uncertainty includes both model uncertainty and aleatoric uncertainty. Given that we modified the setup (X , Y, P, H), the assumption that model uncertainty is negligible no longer holds. The increase in epistemic uncertainty is, therefore, likely reflected stronger in the model uncertainty than it is in the approximation uncertainty. This observation is supported by Heid et al. , who found that, with small data sets, the epistemic uncertainty is dominated by model uncertainty.