When i use two axis, by double clicking on one serie, then on axis label, choosing Plot series in : "Secondary Axis"

My chart looks like :

But now, if i choose (for the FF serie) "Primary Axis", i get :

You can see that the data has been fitted to the ED serie, but the left axis does not correspond to the values of FF serie.

My question is : what is the transformation of data used to fit the other serie ? I just checked that it is not a linear rescalling, so i really don't know what Excel did compute to have such a good fitting of data (actually I want to use the FF serie to predict the ED one, so it could be of very great help to find the answer...)