Basically he fudges the value of the shares in his investment holding company. Each year around the time Forbes are compiling their list (December), the value of Kingdom Holding Co. shares (thinly traded Saudi listed company which he owns 95% - this is essentially his wealth) jump in value, then drop shortly after when sold off again.

So Forbes, noticing the irregular share price movements calculated his wealth based not on the value of the Kingdom Holding Co shares, but instead the value of Kingdom's investment holdings (Apple, Citibank, etc). It's basically an investment vehicle that owns shares in other companies.

Seems Prince Alwaleed is just desperate to be higher on the list and to be known as a the Warren Buffet of the Arab world.

Apparently he also has a copies of finance magazines with his image photo-shopped on covers. Forbes - "The world’s shrewdest businessman" (Steve Jobs removed), "Time "Man of the Year" etc.