The mystery Mr. Jobs took to his grave has finally been solved.
It turns out that Steve quite liked the idea of being different (recall the “think different” slogan so regularly used by Apple) and discovered a perfectly legal way to permanently drive without a number plate.
Steve (or someone close to him) spotted a loophole in the California vehicle laws. Anyone with a brand new car had a maximum of six months to affix the issued number plate to the vehicle.
So Jobs made an arrangement with the leasing company; he would always change cars during the sixth month of the lease, exchanging one silver Mercedes SL55 AMG for another identical one. At no time would he ever be in a car as old as six months; and thus there was no legal requirement to have the number plates fitted.