"X of Ardross" vs. "X Lord of Ardross" dichotomy:
Solution: The title "of Ardross" means the individual in question is the Xth heir to Ardross. Example: William Dishington, Fiar of Ardross was the 6th of Ardross, as there were five people prior to him that were heirs to the estate.
"Lord of Ardross" refers to individuals who actually ruled Ardross. Example: George Dishington was the 5th Lord of Ardross as four people before him ruled the lands he presided over.
The dichotomy comes from the fact that following William Dishington (6th of Ardross)'s death--a death which occured prior to William's ascension to Lordship as his father, the contemporary Lord, was still alive--Willaim's son Thomas took the title Lord of Ardross. As William never ruled as Lord, Thomas is the 7th of Ardross (seventh in line), but only the 6th Lord of Ardross (sixth to rule).