Good observation. Let me explain to you why the two formulas are different. In lesson 3, while estimating the activity duration, we needed to know how many crews we want. Therefore, we consider the crew size in our formula. However, later on in lesson number 5 where we were estimating the required working hours to be assigned in primavera, no need to involve the crew size.
Conclusion: to calculate the required man hours, lesson 5 formula is correct.
I am very happy that you noticed this and raised your question.