In the past, when Ren Xiaosu was learning the duplicated skills, they would be categorized into five levels—beginner, intermediate, advanced, master, and perfect.
The Basic Skill Duplication Scroll could only learn a skill up to the advanced level. Even though Yang Xiaojin's firearms proficiency was perfect, Ren Xiaosu only managed to learn it to the advanced level. It also did not allow the user to learn any superpowers.
If he wanted to continue learning, he had to use the Perfect Skill Duplication Scroll after his firearms proficiency reached the advanced level.
Meanwhile, the Perfect Skill Duplication Scroll would allow the user to randomly copy the target's master-level skill or superpower. If he had duplicated Yang Xiaojin's Perfect Firearms Proficiency and did not have the corresponding Advanced Firearms Proficiency, then this Perfect Skill Duplication Scroll would have been useless.