There were 13 groups descendent from Abraham (Israel), the 12 are as Brandon says (re the descendants of Daniel) but the 13th were the descendants of Levi. Levite's were called to be temple servants, their lives devoted to maintaining the ceremonial rites to ensure the other Israelite tribes could commune with God.
In the same way, a Thirteenth shard may make sense as one the enable commune between the others...
I'm new to theorising, not sure how literally we are to take WoBs.