A question about relationships

[Eslam Elbyaly]

Hi,
In a book I am studying, there is the following ERD and there is something I do not understand.
the scenario, there are volunteers who work for an aquarium. every volunteer have a job to accomplish, and every job need a skill of a volunteer or more to be done.
my question is about "job_numb, skill_numb" foreign key columns in "volunteer_schedule" table. Why skill_numb is a foreign key for "skill_numb" in "job_skill_required" table and not to "skill" table?

2- why is "job_numb" is foreign key to "job" table and not to "job_skill_required" table?