Because he's asking the LLM to interpret those instructions to drive his process. If the skills are poorly defined or incomplete then the process will be as well, and the LLM may misinterpret, choose to ignore, or add its own parts.

Skills are just another kind of programming, albeit at a pretty abstract level. A good initial review process for a Skill is to ask the LLM what it thinks the Skill means and where it thinks there are holes. Just writing it and then running it isn't sufficient.

Another tip is to give the Skill the same input in multiple new sessions - to stop state carryover - collect the output from each session and then feed it back into the LLM and ask it to assess where and why the output was different.