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Here is the official word from Microsoft TechNet, The C:\Users folder is shared when any sub folders from C:\Users\<username>\ is shared. The article is stale and mostly unhelpful, but worth knowing to complete the story.
On checking the properties of the Users folder and the Advanced sharing section of the Sharing tab the Everyone group has full control over the users folder. The folder is also indicated as being in State:Shared. I did not initiate these permissions on the users folder so the only thing I can think of is that when I created a Homegroup that the Homegroup initiated the share. I only created the Homegroup for testing purposes and removed it afterwards.