MQ,
I've not flown the Robin 2160, nor am I familiar with it's layout, but here's a few thoughts, straight from the French TP course notes, circa 1981:
When testing spin recovery in aircraft, particularly in France, all combinations of flight control positions are essayed.
I suspect that on this aircraft, the rudder was found to be the most powerful and thus, the most effective anti spin control. I also suspect that the rudder is blanked by the tailplane when full down elevator is held in a spin (think of where the relative airflow is coming from). In fact, it may be that the blanking is so heavy that spin recovery may not be possible with the stick forward - I must stress that I don't know this for sure, I'm talking generic here. However, if the elevator is held fully back, more of the rudder may be exposed to the airflow, thus allowing the most effective anti-spin control to be employed.
Whilst not a complete answer, I hope the above points you in the right direction.
A