ColorPicking: draw position specifc color. Then read back, but something is not 100% good

Hi

I'm drawing points inside a square. Coordinates range from -0.5 to 0.5. The color of the points depend on his position.
Fragment shader does this:
gl_FragColor = vec4(v_position.x+0.5, v_position.y+0.5, v_position.z+0.5, 1.0);

After rendering in backbuffer I read the buffer:

 byte[] pixel = new byte[4];
GL.ReadPixels(x, _glControl.Height - y, 1, 1, PixelFormat.Rgba, PixelType.UnsignedByte, pixel);

Vector4 point = new Vector4();
point.X = (((float)pixel[0])/255)-0.5f;
point.Y = (((float)pixel[1])/255)-0.5f;
point.Z = (((float)pixel[2])/255)-0.5f;

I want to get back the exact position of the point I clicked on.

The output is correct for position values like 0.1 and 0.2.
But for coordinate 0.15 I get 0.1509804. For 0.03 I get 03333334.

So the precision of this method is not very precies. Does anyone have an idea why this is the case? PixelFormat ? PixelType?

I expect the color on the buffer to be exactly what I specified in the shader. And I expect to read out the exact color from the buffer.
This should be possible I think. Or is OpenGL not precise enough with colors?

thx

regards,
sjoerd222