Thanks for explanations. I agree that in general those lines of the code are not equivalent (for example light_view_matrix may contain some rotation)
But in this particular case the light_view_matrix contains only a translation:
Code :
glTranslatef(-light_position_ws[0], -light_position_ws[1], -light_position_ws[2]);
glGetFloatv(GL_MODELVIEW_MATRIX, light_view_matrix);

The light_view_matrix looks like this:
Code :
1		0 		0		0
0		1		0		0
0		0		1		0
-light_posX	-light_posY	-light_posZ	1

So those lines of code give me the same result when I run the program. If I'm still wrong please give some example.