Fun with numbers

So this happened in class...

Professor: Substituting in these historical values into the model will predict a risk free real return of 5% a year. Unfortunately, in the real world the risk free return has historically been 1%.

Student (raising hand): Philosophically speaking, if this had been physics and they had predicted a gravitational constant 5 times larger than reality, they would have trashed their model. Why are we learning this?

Professor: Well it needs tweaking.