Actually, with respect, I don't agree with all of Jez's assessments. Most of what he said is on the money, however....
The degree of texture and height of bumps vary as well as vary due ONLY to the sharpness of the pull.
But it is there on every helmet. It might not appear so because of a softer pull but it definitely is because of the mold. In fact, they have to be.
The question has still not been answered correctly. Those were nicely thought out logical theorys, but not the reason why.
I still don't believe that anyone will answer correctly as it is a very difficult and technical question. If people are bored with it, feel free to skip it and have someone ask another question. For the meantime, let it serve as the "sword in the stone" to see who can come up with the right answer.