That would also throw me, it seems intuitive from our land based experience that the flag blows backward with moving things as they push through the air. Like flags on cars. But sailing vessels are propelled by the wind, and usually move slower than the wind as well, so the flags would blow with the wind, forward. I'm sure sailors would not get confused about it, but for people who don't have that experience it seems counter-intuitive. Now if you had a sailing ship moving faster the wind, which is actually possible, I guess the flag would blow backward, well at an angle, because I think you have to be moving at an angle to the wind to go faster than it.