Ahimelech was the father of Abiatha also a priest why the confusion in Mark's account?
Maybe Mark misquoted since he was the one telling the story of a story of a story? (As Finkie said)
Maybe sons and father's names were interchangable back in the day?
Maybe father and son priest combo served together?
Maybe the name of the high priest wasn't the point of the allegorical story and not to be nit-picky about the sabbath was the point of the story? Keyword being story.