It could be just a mistake from the kitchen, it happens. However, most of the time, the reason why you get your steak cooked beyond med rare is because a lot of restaurants have a policy where they order their chefs/cooks to cook it beyond medium is to avoid any potential lawsuits resulting from food-borne illnesses. The probability of getting a food-borne illness from a medium rare or medium steak is very low because any ecoli or such, if present, is going to be on the outside of the meat which is seared off during the cooking process. However, restaurants usually want to avoid any possibility of customers getting sick from their food so they tend to overcook things.