That'll be the side effect of "Brittany Ferries flying the Jolly Roger" wrt fare prices. My group used to always do Pompey-Caen but at £350 or whatever for an overnight with cabin and a return on Monday afternoon, it just makes more sense to go Dover-Calais early on Thursday, even though it adds hundreds of miles to the trip (and a Premier Travel Inn on the Wednesday night too...)
For our trip Brittany Pirates wanted nearer £700! Hence the drive 60m up the coast to Newhaven for a £240 trip. I know it's a longer drive but we ain't going to use and extra £470 in fuel!