Why deviates in opengl the gl_FragCoord.z from the depth buffer one?

[PowerNow]

Hi,
I have been trying for some time to find a way to read in opengl the depth value for a particular mouse coordinate (x, y). Everything works fine on win10 with opengl 4.x, but not for opengl es 3.x

My approaches:

1.) glReadPixels() does not work on openGL es for depth buffer
2.) ray cast is not suitable because I work with a large terrain model
3.) subsequent method would suffice, but unfortunately too inaccurate, also on win10 but why?

#version 420
uniform vec2 screenXy;
uniform vec2 screenSize;

out vec4 fragColor;

void main(void) {

if((int(gl_FragCoord.x) == int(screenXy.x)) && ((int(screenSize.y) - int(gl_FragCoord.y)) == int(screenXy.y))) {
    fragColor.r = gl_FragCoord.z;
    } else {
        fragColor = vec4(1, 1, 1, 1.0);
    }
}

I submit the mouse xy coordinates to the fragementshader (screenXy). If the clicked pixel is in the row, I write the depth value in the color buffer. This works, but the value gl_FragCoord.z and the one from the depth buffer are not exactly the same (I know this one from the depth buffer is correct). Although gl_FragCoord.z and the depth buffer value is float, and so I think 32bit.

GLfloat zd; // from depth buffer
GLfloat z[4]; // from color buffer
m_func->glReadPixels(xy.x(), m_pFbo->height() - xy.y(), 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &zd);
m_func->glReadPixels(xy.x(), m_pFbo->height() - xy.y(), 1, 1, GL_RGBA, GL_FLOAT, z);

Reasons:
1.) the deviation occurs through an internal type conversion, but where?
2.) because the GL_DEPTH_TEST is executed after the fragmentshader gl_FragCoord.z is not the closest one (to the camera), but which is saved in the depth buffer. So it would also make no sense to save gl_FragCoord.z in a separat Frambuffer, because its not the correct value.

Can maybe someone help me and solve the knot, because I can not find any other explanation?

Here some measured values:

zc  0.984314
zd  0.985363

zc  0.552941
zd  0.554653

zc  1 -> extremly critical
zd  0.999181

Thxs in andvance...