Not sure how to explain this off the top of my head, but I'll try, and send a tracing from a properly researched book, and not a public genealogy source later.
There was more than one Humphrey DeBohun, and two had royal descendants. The DeBohuns originated in France and may have been distant kin to William the Conquerer. The first Humphrey came to England during the Norman conquest. He was given land and titles for his support of William. By making brilliant marriages, the DeBohun men gained more land and titles by right of their wives. One title was Earl of Hereford. Humphrey, the 4th Earl of Hereford was raised in the court of Edward I. Edward married his daughter, Princess Elizabeth to Humphrey with the stipulation that the DeBohun land and titles would revert to the crown. Humphrey and Elizabeth had eight children I think, and this is where the royal line starts....through her.
The titles Earl of Hereford and Essex, etc, were passed to their oldest son who, died childless. His brother William De Bohun, Earl of Northhampton( who was the fifth son) had a son Humphrey, who became the 7th Earl.
He and his wife, whose name escapes me at the moment, had two daughters. Eleanor, and Mary, but no sons. Thus ends the direct, royal, Humphrey DeBohun line.
I totally agree with Jonas Wilson above, who states that people fabricate dates and merge generations. I see it all the time on genealogy sites and blogs.
There is some good news though. The last Humphrey, the father of Mary and Eleanor, had a sister named Elizabeth. Her granddaughters are probably the direct ancestors of everyone here......including me. Please let me know if you'd like more info.
Ellie