Yes thats the case. Of course the LWB and Roof rack bump up the price. When you think that what you are really buying is space. A car is the same size if it's just got a driver or if it's full with four or five occupants. The same applies to a cabin. The question is do they make a profit on the crossings that are not popular? If they do then IMO they are not really playing fair to Jack the prices up on the popular crossings.
I don't know but I've always thought that they top up the load on the Ferry with the unaccompanied trailers you see on the dock side. So even the less popular crossings are full as far as car and trucks and trailers go.
So
The thing is, they have a captive market. If folk are willing to pay higher prices for a limited number of ferry and tunnel places, which are the only way that most folk are going to get to Le Mans, then as places run out the price goes up accordingly.
It's basically supply and demand.
The ferry companies can't be unaware of the massive spike in business they get during race week, just a simple look at their statistics will have shown them that.
So, supply decreases in relation to increased demand, and as that happens the prices go up accordingly.