Why is polar curve of a glider dependent on flight load? This is my first question here.
I'm just a glider in education phase and not yet a pilot, but because I'm also physicist, I'm interested in some details more than other people. 

My question is simple:

I know what a polar curve is. Why is this curve dependent on the wing load?
For my knowledge in aerodynamics the polar curve is a property of the airplane/wings, but not dependent on flight state. Otherwise I know from my flight instructor, that gliders are often filled with water to increase weight, which gives a better gliding ratio.

What I mean you see on this curve: why is the curve changing with wing load?
My only answer would be, that wings are flexed differently under different loads and that the different curves arise from (slightly) different geometries. Is there something else I have overseen?
Oh god- I made a terrible mistake: While $C_{lift}$ and $C_{drag}$ are really independent on weight, this is (of course..) not the case for the velocity polar curve. The two curves scale perfectly with weight and are just subject to a simple transformation. No magic behind and completely transparent.

Sorry: Question Solved ;-)