I think it is just the angle of view. Probably taken fairly close with a wide angle lens, accentuating the dome??? Either that or the picture from the Seatool website is of a pre-production model and the actual production dome is smaller.
I was a bit surprised at how compact the dome port was when I first saw it. I picked up the box and thought it was mis-marked when it had a dome port label on it. At the moment, the only Seatool ports for this housing are two flat ports, for the 45mm macro and 14-45 zoom, and one dome port, for the 7-14.
I suspect they will have a dome for the 8mm fisheye by the end of the year or so, but for now, only one dome port. Truly.