The Operational Momentum (OM) effect refers to the tendency to overestimate addition and underestimate subtraction outcomes during mental arithmetic, however, whether OM effect arises from multiple competing sources remains unclear. To explore the mechanism of OM effect, based on the theory of operand encoding in Triple Code Model (TCM), we divide operands into magnitude encoding and verbal encoding, and compare the differences of OM-congruent probe and OM-incongruent probe at behavioral and electrophysiological level under different encoding conditions. We found that in the magnitude encoding condition, participants exhibited a stable OM effect, and OM-congruent probes elicited a positive P3 component (250–350ms) in the parietal lobes, while OM-incongruent probes triggered a more negative N2 component (150–250ms) in the parietal lobes. In the verbal encoding condition, participants exhibited a reserved OM effect and OM-incongruent probes elicited a more negative N2 component (150–250ms) in the frontal lobes. These results indicate OM effect arises from multiple competing sources of bias in mental arithmetic, which is consistent with the assumptions of the AHAB model.