Both answers above are right.
There is a difference (by definition) between a SSM and a PM. However, many organizations use all of the above definitions for what would conventionally be called a Purchasing Manager.
Why? Look at the US average and median pay scale for a "Purchasing Manager" in the Department of Labor's Wage information. Now, change the title to something different, and it provides the company with leverage to deny that the Purchasing Manager is an actual Purchasing Manager...he is a "Sourcing Manager" or whatever the hot term for the day is.