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Understanding bit error rates of magnetic tunnel junctions (MTJs) is critical for designing reliable spin-transfer-torque magnetoresistive random access memory. In this letter, we study the write error rate (WER) of two types of in-plane MTJs with the same film stacks except for the free layer and the capping layer. By comparative analysis of these two MTJ splits that show significantly different WER characteristics, we find that reducing average switching voltages does not necessarily improve WER slopes, resulting in decreased write margins for a sufficiently low WER requirement. Various magnetic measurements suggest that WER slopes and slope asymmetries are more strongly correlated to spin torque efficiencies rather than thermal stability factors.