Unusual stepwise increases of efficiency in fluorescent organic light-emitting diodes (OLEDs) was reported, which has not been observed in phosphorescent devices. However, the root cause of this phenomenon is unclear. Herein, we investigate the correlation between non-linear efficiency characteristics and the leakage current in conjunction with exciton formation in the electron blocking layer (EBL) as a function of voltage. Hole and electron transporting layers with different injection characteristics were selected to control the leakage current into the EBL. It was revealed that a significant amount of excitons were formed in the EBL. It was discovered that the degree of variation in the efficiency curve was directly related to the amount of leakage electrons from the emitting layer (EML) and the number of excitons in the EBL. Moreover, the exciton formation zone (EFZ) gradually shifted from the EBL into the EML as the voltage increased. After device degradation, the stepwise shape of the efficiency curves was significantly deformed, strongly implying that contributions to efficiency by excitons in the EBL vanished. Thus, the unusual stepwise increase in efficiency can be fully attributed to exciton formation in the EBL and shifting of the EFZ from the EBL to the EML.