A swimmer's mile

[Dooley67]

I had always thought that swimming a mile meant swimming 36 laps or 72 lengths in a 25 yard pool which is 1,760 yards. However, on the bottom of the summer pool schedule at my Y, a "swimmer's mile" was listed as 66 lengths which amounts to 1600 yards. What is the reason for the disparity and which does one adhere to when swimming a mile?

D67