Good question
@MorningAngel add to that the fact that most people are pretty bad at knowing what costumes were worn when (Including the props depts of various costume dramas!
) and that they may be somewhat shocked at the appearence of an apparition and I guess there is a lot of speculation involved.
Legends also seem to be involved in ordinary histories as well, I imagine that Mary Queen of Scots must have slept in a different bed every night of her life if you believe the number of historic houses claiming she slept there!
Interesting theory
@Steven; ghost stories (at least those originating before about 1930) do seem to relate to a somewhat weird version of actual history given the numbers of randy monks and nuns, secret tunnels, burned witches, walled up nuns and lords of the manor codemned to forever ride headless horses up driveways etc. I've often wondered why certain periods of history seem to attract ghost stories whereas others don't (How many ghosts at Gettysburg, how many at the Somme?) Using the stories as a way of manipulating a general view of historic events or accepting establishment views is a fascinating idea.
Off topic - I'd recommend "Mud Blood and Poppycock" by Gordon Corrigan. It is controversial but dispels some popular ideas about that war. Possibly because of WW2 we tend to forget the outrage felt in Britain and elsewhere at the German invasion of France and Belgium in 1914, possibly greater than in1939 when Nazi atrocities were not as well known.