I wouldn’t call it a model, more like a “design pattern”

There is a difference between productivity and activity, you can be 100% active at work all day, yet 0% productive. Imagine you work on a project for 6 months and then the manager decide to drop the project. You have been unproductive for 6 months, doesn’t mean you were slacking off, but in the end when we calculate the productivity of developers, it is lower because of this.