WebGL/GLSL vertex shader collapses 9 points into single point when I set x coord of gl_Position to zero

Question

I do not understand why the following two shaders produce different results when I render a vertex buffer with x coordinates equal to zero:

First:

attribute vec3 position;

void main() {
  gl_Position = vec4(position.x, position.y, 0.0, 1.0);
}

Second:

attribute vec3 position;

void main() {
  gl_Position = vec4(0.0, position.y, 0.0, 1.0);
}

The result of the first is a line of nine dots. The result of the second is a single dot.

I'm drawing the following vertex array as GL_POINTS:

0.0, -1.00, 0.0,
0.0, -0.75, 0.0,
0.0, -0.50, 0.0,
0.0, -0.25, 0.0,
0.0,  0.00, 0.0,
0.0,  0.25, 0.0,
0.0,  0.50, 0.0,
0.0,  0.75, 0.0,
0.0,  1.00, 0.0

Here's the VBO preparation calls:

var a = new Float32Array([
    0.0, -1.00, 0.0,
    0.0, -0.75, 0.0,
    0.0, -0.50, 0.0,
    0.0, -0.25, 0.0,
    0.0,  0.00, 0.0,
    0.0,  0.25, 0.0,
    0.0,  0.50, 0.0,
    0.0,  0.75, 0.0,
    0.0,  1.00, 0.0
]);
gl.bindBuffer(gl.ARRAY_BUFFER, b);
gl.bufferData(gl.ARRAY_BUFFER, a.byteLength, gl.STATIC_DRAW);
gl.bufferSubData(gl.ARRAY_BUFFER, 0, a);

Here's the draw calls:

gl.bindBuffer(gl.ARRAY_BUFFER, b);
gl.vertexAttribPointer(p.position, 3, gl.FLOAT, false, 0, 0);
gl.enableVertexAttribArray(p.position);
gl.drawArrays(gl.POINTS, 0, 9);