Joe,
After the last rebellion, lots of things Scottish were officially "repressed" by the govt. in London. (read "outlawed") Not until Victorian times were the tartans allowed (military units were allowed certain govt. patterns) and by that time, many of the original patterns were lost. They made up new ones based in fact and fiction.
Additionally, chemical dyes were replacing the vegetable dyes used in the 1700's, so this would further alter the apperance.
A cool subject! I am by no means an expert. I do enjoy painting a good kilt or trews now and again!