They are the same if the flow is steady. But if the flow is unsteady (an airfoil changing angle of attack with time, for example), they can be different. The National Committee for Fluid Mechanics Films from 1961 illustrate this very well - go to 8:36 of this video (ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-mh4hZbNMEfo.html) to see a nice experiment demonstrating the differences in the case of unsteady flow.
