Why is the result from the Race Power Calculator different from the max power for that duration indicated by my Model Curve on the Power Duration Curve?

The Model Curve and the Race Power Calculator serve two different purposes.  Both of these tools use your last 90 days of training data. However, the Model Curve focuses on your theoretical capability by mapping the potential contribution to the power of your bioenergetic systems. Race Power Calculator focuses on the definite capability that you have previously displayed in your training data.

As you log more max efforts across your Power Duration Curve, you can expect that your definite capability determined by the Race Power Calculator will more closely match your theoretical capability determined by the Model Curve.