As a core electrical parameter of high-speed interconnection links, the measurement accuracy of differential impedance and common mode impedance is critical to ensuring signal integrity. However, the fixture-induced pull effect causes the measured values to deviate significantly from their true values, which has become a prevalent challenge in the high-precision testing of high-speed connectors for Ethernet systems. Most existing studies rely on single-factor analysis methods, which fail to reveal the synergistic mechanism of tolerance and electromagnetic coupling. This paper proposes a dual-factor pull model that integrates tolerance variation and coupling strength. By introducing a tolerance variation coefficient and a coupling coefficient, a quantitative method for the pull factor is established, enabling accurate quantification of the pull effect. Simulation and experimental results indicate that the proposed model can effectively characterize the synergistic interaction mechanism between tolerance variation and electromagnetic coupling. Under the strong coupling condition, the deviation rate of impedance testing for the fixture with 3% tolerance can be controlled within 16.8% after being accurately quantified by the model, and the error between the model prediction value and the measured value is within 3%. This study provides a theoretical model and experimental basis for high-precision impedance testing, and is of great value for improving the reliability of performance evaluation for high-speed connecting equipment.