First, I added 2 red lines meant only to compare the steepness of the shoulders. The upper line is steeper. The steeper line means better separation of tones, it really is that simple.
Not just the shoulder is flatter, the whole curve is flatter (compressed) with less development. Sure you then use harder paper but there is no gain in detail printed.
Second, is the captain obvious moment. While the theory can be compared using the Tri-X curve, the real shape of the HP5 curve is different. Its shouldering characteristics are different.
And the basic shape of the shoulder isn't absolute, it will change depending on your techniques. It can be manipulated. Below is a good article.