Why do simple wafer fab models fail in certain scenarios?

Previous work has proved that simple simulation models are sufficient for analyzing the behavior of complex wafer fabs in certain scenarios. We give an example where the simple model fails to accurately predict cycle times and WIP levels of the complex model. To determine the reason for this behavior, we analyze the correlation properties of a MIMAC (Measurement and Improvement of Manufacturing Capacity) full fab model and the corresponding simple one. It turns out that the simple model is not capable of capturing the correlations in an adequate way because there is lot overtaking (passing) in the simple model while almost no overtaking can be found in the complex counterpart