Why does the vertex shader work so differently on different devices?

Question

Qt

I have a vertex shader that multiplies a vertex by a normal and then projects it. Its task is to triangulate lines. Normals are calculated on the CPU. Line width is set by u_width

#version 100
attribute vec4 a_position;
attribute vec2 a_normal;
uniform mat4 u_proj;;
uniform float u_width;

void main() {
 vec4 delta = vec4(a_normal * u_width, 0, 0);
 vec4 new_pos = a_position+delta;
 gl_Position = u_proj*new_pos;
}

The result of working on a desktop under windows, on an android tablet with support for opengl es 3.1, as well as in a browser (webGL) is exactly the same. Everything works as it should. This is what the result looks like. Lines of arbitrary width are obtained.


But (!) I have an android phone (based on intel Atom and opengl ES 2.0) and everything works completely differently on it. Namely: the value of u_width does not change the thickness of the line, but simply scales all the points, making the lines longer, but not wider. The picture below shows the same test scene.

I can achieve exactly the same result on the desktop by simply scaling the matrix by a number equal to the line width instead of multiplying the normal.matrix.scale(width,width,0);
It turns out that on this device (phone based on intel Atom) the value of u_width simply scales the matrix. But how? And why? Maybe Qt is doing something with my shader at application compile time?