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Resistive-switching memory (RRAM) may provide a scalable, low-power alternative to Flash memories for sub-10 nm technology nodes. Due to the atomic-size conductive filament (CF), however, switching variability due to few-defect migration is becoming one of the main concerns for RRAM scaling. This work addresses set-state variability in HfOx RRAM. Our study shows that variability increases at small CF size, which is quantitatively explained by defect-number fluctuations within a Poisson distribution. The set failure at relatively large CF sizes is then shown for the first time and explained by complementary switching (CS). Advanced program-verify schemes are needed to improve the distribution shape and allow reliable RRAM operation, e.g., in multilevel cell applications.