I think I figured it out. If you look at page 56 of game specific drawings. There is a marking for 60 degrees. 60 is the angle between the 4 corner posts, and the horizontal plane.

68 is the angle between the horizontal members and the corner posts in the plane of the side.

The angle between the horizontal plane and the corner posts is different from the angle in the plane of one of the 4 triangular sides between the horizontal bars and the corner posts.

Visual aid two pencils in an L shape tilt the vertical pencil so it is 30 degrees off the table, its angle to the horizontal plane is 30, but the angle to the other pencil in the plane they share is still 90.

Additional hat tip to ZehP who did the math while I did my thought experiment: