Unfortunately, I don’t have a great answer for you, other than that was the way Spencer built this particular model. I will point out that the Catch Up feature in the model you reference is a profit catch up, not an IRR catch up. And as such, is most common in scenarios where the GP doesn’t contribute capital.
If you’re looking for a model with an IRR catch up feature that applies in scenarios where the GP also contributes capital, you might check out either of these models:
https://www.adventuresincre.com/excel-real-estate-equity-waterfall-model-monthly-periods/
https://www.adventuresincre.com/real-estate-equity-waterfall-model/