Incorrect Order - drive thru

Please can you tell why is it that the person taking down the order in the driver through always get my orders wrong... Is it so difficult to get a large cup of coffee when you tell them you want a large cup of coffee and is it so difficult for them to understand that when you say you want a sausage and egg mac ****in it must actually have a sausage and an egg in it and not jus an egg... this was last week... today a placed an order for a sausage and egg mac ****in and got only a sausage...