The castle has been a prison for about fifteen years (~50 if you count it's time as a home for "bruised" boys or "bad girls"). The tale of the ghost has been a part of its story for at least a couple hundred years, possibly as long as ~300 years, if the origin of the myth is the Lyons family as theorized. Real or not, it has at least as valid a claim as relevant information as Huntly's current status as a low-security prison.  

 Granted, I can't see how a Lyons female (occupation 1614-1775) jumped from a tower built by George Patterson in 1776...