Spread changes fine tune in Z3TA+ 2

When I use the "spread" (in the osc-section), I notice that the fine tuning changes. I have checked this in my tuner named StroboSoft.
When I have "spread" 50%, I must have the "fine tune" to 25 for it to be the right tone in my tuner.
Mode = Multi, Free
 
I do not understand why I need to change the fine tuning?