A viscosity anomaly in immiscible polymer blends takes place when the viscosity of its melt falls below the value calculated from the linear additivity rule applied to viscosities of the components. This work examines interfacial origins of the anomaly through a parallel analysis of morphology and flow rate of polypropylene–polystyrene blends. Experiments were conducted with a capillary rheometer using the method of layered blends. The method allows a convenient morphological characterization of collected extrudates. The combined knowledge of the morphology and flow rates enabled a quantitative analysis of flow patterns during the extrusion. Strong circumstantial evidence supports, but does not confirm, that interfacial effects contribute little, if any, to the observed viscosity anomaly.