Obviously the pricing is geared to encourage people to fill their cars. Interesting in this country how we always discourage things through punishments rather than encouragements through incentives - not a criticism of the club, just the way Britain works.
Here's a radical idea: car with one person in £20. 2 people in - £15. 3 people in - £10. 4 or more - a fiver.
Of course, it wouldn't raise as much money but as it's all about sustainability really ;-)
But, it works out that way anyway, without the club losing money. If more than one person uses the car and shares the cost of parking a group of four pay less than a single driver.
If the club charged differently based on how many were in a car it would complicate issues for them - what if someone will have 4 people for saturday games, but two them can't make midweek games, unless they're on a monday and half way through the season tuesday gam gets moved to a monday for TV - how much do they charge? How do they ensure that four people are in a car every week on this season ticket? Stewards at the parking ground might be able to keep an eye on it, but the complications around payment for under occupancy in the car, or refunds for over occupancy would be a hassle, no guarantee some guy has driven solo to the ground and got a friend to come in the car to fill it up but don't go to football?
Charge by the car and let the driver spread the cost through his companions and each of the individuals pay less than if they each drive their own car, thus get their reward for sharing.