It's just a matter of perspective. When the 3-dimensional world is compressed down to 2 dimensions, the vantage point determines the apparent distribution of the objects in the picture. The pictures above showing the supposed conical distribution are showing either
1. Only a part of the skyline, where the pattern holds locally
or 2. A particular angle that places the tallest building at the center of the view with a smooth height gradient on either side
Seattle's skyline is decidedly non-conical in arrangement, despite how it appears from that and some other angles. The tallest building is near the southern edge of downtown. View the city from the east, or more easily, from the west, and a very different distribution is revealed. This view could have been used in an Alltel ad (maybe it was), with the ascending towers resembling signal bars on a cell phone.
(photo by me; yes, it's an old one)
I think what's really going on is that angles that show skyscrapers in an apparently conical grouping make for pleasingly stereotypical skyline photos that work well for postcards and such. This could be because of their resemblance to the classic Lower Manhattan skyline as seen from the harbor, which had the conical distribution with the tallest buildings in the center and progressively shorter buildings farther out. It's not surprising that the cliche skyline views of other cities evoke the form of the original iconic skyline.