Discretionary decisions are integral to the Criminal Justice System, regardless of branch or stage. Often, structured interventions are introduced in an effort to control or guide discretion use. In the USA, grid-based sentencing guidelines are commonly used to channel sentencing discretion. However, despite their presence, research consistently shows that, net of legally relevant factors, significant racial disparity persists in sentencing outcomes under guideline structures. Although judicial and prosecutorial-based explanations for this abound, how the structural components of these guidelines (such as the offense seriousness axis) and the discretionary decisions related to them might contribute to this disparity have seldom been explored. Using federal sentencing data, a data-partitioning strategy and multilevel multivariate analyses, this study examines whether there are significant differences in the federal offense seriousness calculations for black and white defendants. The results suggest that Federal Probation Officers (FPO), in an effort to meet their diverse responsibilities in the time allotted, use heuristics when making offense seriousness determinations. This furnishes a previously neglected explanation for the persisting differential effects and highlights the utility of heuristic mechanisms in explaining both discretionary decisions and apparent disparate treatment by race or ethnicity in the criminal justice system.