Interesting. FWIW, you can clearly see that the horn in Bennett's U103 is different than the picture you provided. Your picture clearly shows a more common CD type diffraction slot, where the one in Bennett's U103 is slightly different than normal. You can also tell that your horn is different by looking at the mounting flange. The flange of course does not affect the pattern of the horn, I was just pointing it for the sake of showing that maybe your horn picture is from an earlier design.
What causes the HF to follow the sides of bell are the flat areas immediately adjacent to the throat hole, usually left & right of the hole and adjacent to the curved walls, just like your picture. However, you can see in Bennett's U103 horn that the larger flat areas are positioned in the vertical along with the curved sides. This is a 90 degree shift from normal horn geometry basics, and against B&Cs own recommendation.
Whatever though, the good thing is this horn is totally rotatable, so whatever works for you.