Why do SQL Server auto-created statistics sometimes include the primary key?
I have spent a decent amount of time searching for an explanation of the different naming conventions on some of my own databases, and haven't found anything definitive.
That being said, in both our cases we are dealing with very old databases that have undergone many upgrades over the years. My best guess is that at some point Microsoft switched the auto-created statistics to use an incremental alphanumeric pattern instead of using partial column names (somewhere around SQL Server 2000 or 2005?). This may have been done to reduce the potential for naming conflicts or to better handle the renaming of a column.
I haven't seen the Primary Key be added into auto-created statistics, but that might also be an artifact of a very old version.