Its just speculation given that i havent used the ability myself. However id say that improvise doesnt reduce the chances of the element going chaotic, but rather reduces its actual effects.
Unstable elements either reduce progressor quality gained or increases dura lost on a succesful or failed synth.
Which one of those 3 it is that is reduced or increased depends on the type of element.

Now thats where the speculation comes in: Improvise reduces the effects that an unstable element has on reducing progress/quality or increasing dura loss.