Unless McDonalds has done an incredible job of increasing their margins threefold since I worked there (left in 1995), there's no way their net margins are so high. Our best store made 10% net, and that paid for a lot of the crappier stores.
That's the parent company's financial figures, which include profits from franchisee's. This inflates their earnings. A typical store (franchisee or McOpCo) clears about 10% net of their sales. For a McOpCo store (one owned directly by McDonalds), this is not bad at all, since they don't have to pay a franchise fee. This means they're clearing 5-8% more. But for a franchisee, they pay a franchise fee of around that amount. So take home is not as extravagant. 90% of McDonalds are franchised, so the parent company's earnings are largely composed of franchise fees, not sales margins.