Ok, this is how I see it:
Boresight is a direction, a line, from the antenna where the signal (reception) is strongest (That is the optical axis of the antenna. Further you get from the boresight, weaker the signal will be). In wide mode when you lower the boresight -3 degrees, reception from above the boresight is still good enough to detect a target. But in narrow mode, beam is narrower (of course) so now when you lower the boresight -3 degrees 'top most part' of the beam, capable of detecting targets, is about 2 degrees under the horizon.
Ive included my own picture, RED - boresight, Green - target, BLACK lines - beam width. This is how I think this is happening, if I am wrong Hpasp will surely try to correct me.



p.s. It would be alot easier to explain in person, as Hpasp said, because of all the hand-waving and face-making you can do.
p.p.s. Oh, and the shape of the beam is not exactly a triangle, it is more of a ellipse. This might clarify my post a bit.