Economic transmission planning heavily relies on the accuracy of the quantitative analysis using market simulation tool, and it therefore requires benchmarking the simulation tool and model against historical actuals. Facing the technical challenges of such analyses, there is a lack of systematic approach and standard in doing such analyses in the industry. In that regard, this paper presents a thorough analysis that has been recently conducted at MISO which many stakeholders considered by far the most comprehensive and profound study of its kind. The analysis not only identified and categorized various impacting factors that caused discrepancy between model predictions and market actuals, but also quantitatively measured the impact of the major impacting factors. The improved results from applying the best modeling practices are presented to demonstrate the effectiveness of this analysis, which will bring great value to other entities in need of such analysis.