Systematic judgements occur less frequently in source judgements because they are slow and require a lot of conscious effort. Errors occur due to a misassignment of the weight of certain aspects of memories: assigning high importance to visual information would mean that having poor details of this aspect would be cause for an assumption that the event did not happen or was imagined.
Systematic assessments are less common in source evaluations because they require a substantial amount of deliberate thought. Errors arise from misassigning the significance of certain memory components: assigning high importance to visual information would lead to an assumption that the event did not occur or was imagined if there were poor details of this aspect.