realskiers

3 centimeters is certainly more understandable.  That is only slightly more than an inch, and also happens to be the range of movement that Head builds into their railflex plate, since they have a 0 position, a +1.5cm mark, and -1.5cm mark.  Being 3cm forward probably wouldn't have made the ski dive in soft snow but could easily cause the symptoms you saw on a firmer surface. 

If you have enough adjustment, I would try skiing them in a few different positions even more than 3cm back and see what you like.  The second biggest feature of rail based skis, in my opinion, is the ability to move the bindings around and see how they feel without having to redrill or remount any hardware.

good luck.