While the thing is still pistonic (so, up to 200 Hz then
), if the long axis of the ellipse is vertical then the thing will have slightly tighter directivity in the vertical plane, than in the horizontal.
A rather hopeful marketeer might claim that this means that the design offers increased radiating area (true) and hence better LF response (true) whilst pushing the frequency at which horizontal directivity narrows, up to a higher value than would be the case if that area were to be achieved using a circular cone.
In practice, by the time you get up there, the thing is probably starting to vibrate modally and so directivity gets a lot more complicated anyway. Hey ho.
(Btw - anyone looking at Borwick would do well to find ed. 2 in my view - quite a lot of useful math detail was lost in ed. 3, including chap 1 by the great R.D. Ford).