So, I thought about this some more, and I figured out what was nagging me about it. This isn't a map of the whole Shattered Plains; it covers only about a quarter of them. That means the lines here converge around 1/8 the way through the Shattered Plains, not in the center.
The answer is right in the caption above. The Tower is the place at the end of Alethi reach, shown here at the very right edge of the image (the "largest plateau of the area"). Bad battles were fought over it, with the Parshendi generally having the advantage of arriving first. They were also quicker to travel over this terrain, which makes Stormseat at least twice the distance shown on this map. That would then mean the lines that appear to converge nicely in a radial manner above should instead be much more unidirectional, West to East.
Hate to rain on the parade, but I don't think it can be correct.