The Lord Lyon, King of Arms has recognized a Chief of the Name and Arms of Henderson. The existence of many roots of the Henderson Family Tree is acknowledged. Even if you trace your pedigree to an ancestor with Arms in England the root search brings you into Clan Henderson, a family which includes all Hendersons -by any name.
Your Chief's surname is Henderson of Fordell, but he is the Chief of the Name and Arms of Henderson (not Henderson of Fordell)! As Chief of the Name and Arms, that means that if you have the family name of Henderson, he is your Chief.
Why was Dr. John William Philp Henderson of Fordell chosen? Probably for two reasons. Of the several families we can identify to lands in Scotland, only the Hendersons of Fordell can claim continuing status as a clan. Also because Dr. John is the only one who has presented an effective case to be the Chief. His recognition is based upon his relationship to the original Henderson grant of Arms circa 1544, not upon the first instance of a child being called the son of Henry.