Thank you all for the replies!

I think wvr is right about thrust line - wing line difference: once I heard a top pattern pilot giving this explaination.

But I can't figure out WHY such a difference would need differentials. Also, my pattern plane has a quite low wing placement and still need "positive" differentials, not "negative" (I mean that the upward movement must be greater than the downward, in order to do axial rolls)

So it seems the right explaination is missing...

For those of you interested in the topic, I posted my question to the Aerodynamic forum too.