There might be a couple reasons. There might be another position open in the company that needs a different skill set, but HR can save some money combining the two postings together. Another reason might be that HR feels it will get too many applicants if they just put a couple needed skills, so they add some others to help narrow down the number of people applying. It sounds stupid, and well... it is.
My advice to people applying for jobs is to not feel discouraged from calling a company if their posting says they need 10 different skills, but you only have 4 of those skills.
