Weird redstone priorities

What I expected to happen:
That the command blocks would go off in order (1, 2, 3, 4)

What actually happened:
The command blocks went off in the wrong order (2, 1, 3, 4)

How to reproduce:
Just build what you see in the images, and then put buttons across the front, next to the first one, and push them one by one. You'll see the first 3 go 2-1-3-4 and the 4th one goes 4-3-2-1.