The average boiling points (ABPs) of narrow boiling range oil distillation cuts are important in predicting thermodynamic and physical properties of oils. Due to convenience, simple batch distillation methods, either at atmospheric or reduced pressure, are often used to separate shale oils into fractions, including narrow boiling range fractions, and it has been attempted to calculate average boiling points directly from the distillation data. Using wide industrial shale oil fractions from Estonian kukersite oil shale and based on ASTM "Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure" (ASTM D86), this paper is aimed to find out how much the average boiling points determined directly from distillation, as an arithmetic average of the initial and final temperatures of the thermometer during fractions collection, differ from actual average boiling points (Lambda ABPs). The actual average boiling points of narrow boiling range oil fractions, pre-prepared by the same ASTM D86 distillation, were measured afterwards using a recently developed thermogravimetric analysis (TGA) based experimental method, which requires only about 20 mg of sample. The study indicated that AABPs were always lower than the respective average values determined directly from ASTM D86 distillation data.