The model kept coming in under standard. On a frozen-foods line we were modeling, simulated output sat consistently below the plant’s standard rate — not drifting, not noisy, but short by the same amount every single run.
A constant gap is a signature. A rate error would scale with run length; this did not. The missing product was sitting in the spiral freezer — work-in-process that has entered the line but not yet come out the other end. The plant’s standard, computed as a rate, has no way to represent product in transit. The simulation does, because it models the freezer as a real stage with real residence time.
Neither number was wrong. They were answering different questions, and only one of them accounts for the line actually filling up. That distinction is worth real money at changeover and startup — and it is completely invisible to a spreadsheet standard.
Come to the booth and we will show you the run.