I have been thinking about Jorge's point of view all day and am now wondering if the photo of the Duchess at the recent gala presents us with a slightly different view because of the effect of the back chain as Mauriz described and the slight angle at which the Duchess is wearing the tiara. Also, perhaps, part of the central element hangs over the tiara frame (and is obscured by the Duchess' hair) making the gap or distance between the edge of the central element and the bottom of the side elements look smaller.
I have done a very basic attempt at a side by side comparison by rotating the image of the jewel as a necklace, and I think that the central element is attached to the side elements at the same point in the jewel as a tiara and as a necklace, but I could be mistaken. Everyone knows my eye is not brilliant!