I first noticed this a few years ago, but I wasn't entirely sure where or why, but it's definitely become clear that the Dhulbahante lineage has a piece missing.
Consider this: There were 4 men that tradition and various chronicles state were alive during the time of Ahmed Gurey that I have lineages for:
Amir Nur, the Marehan sultan of Harar who succeeded Ahmed Gurey as leader of Harar
http://www.abtirsi.com/view.php?person=5488
Hirabu, a Marehan chief mentioned in the Futuh al-Habasha
http://www.abtirsi.com/view.php?person=1652
Sultan Umardin, the young Sultan of Adal who Ahmed Gurey put on the throne
http://www.abtirsi.com/view.php?person=883
Garad Shirshore, who was a contemporary of Ahmed Gurey
http://www.abtirsi.com/view.php?person=441
All these men lived about 14 generations ago, in the middle of the 16th century, but while the first three count about 25 generations to Darod, Garad Shirshore counts only 11. In fact if you remove the generations between Harti and Kombe (which Dhulbahante do not include) there are only 9.
There are something like 14 generations missing between Garad Shirshore and Harti