It could be explained by not being able to go point a-b and thus having to travel a route around an area of space...thus Making the run in x parsecs.
It has been explained as such! I guess I am the resident Star Wars uber nerd..but the reason why, at the expense of repeating myself, is that in the EU of Star Wars Kessel sit in a region of space called The Maw which is littered with black holes. There are specific routes you have to take in order to avoid them, or you do as the Falcon did with Solo and risk it by taking short cuts.