The main sites of the edge of the East Volcanic Zone are distiributed quite regularly. Here is why:
The line is actually down-welling, marking the eastern edge of the East Volcanic Zone, which is also a rift zone. When marking the line between the N-American and Eurasian Tectonic Plates, most scientists would draw a line along the middle of the East Volcanic Zone. With the knowledge about the convection rolls, this line is often chosen to show the division line, because all the rifting is actually due to the pulling effect between the N-American Plate and the convection roll rotating into the other direction.
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