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It seems to be broken actually, or at least there is an inconsistency between what is displayed in the budget window and what is observed in the piety tooltip.
The budget window assumes the church donations slider to be from -1.0 to +1.0, but the actual piety modification is from +0.0 to +1.0, so the middle position in the budget shows +0.0, but shows (and gives) +0.5 in the piety indicator.
 
Veldmaarschalk said:
Or you have 'traits' that give you extra piety

Traits show up in the calculation.

Kick-Asterix said:
The budget window assumes the church donations slider to be from -1.0 to +1.0, but the actual piety modification is from +0.0 to +1.0, so the middle position in the budget shows +0.0, but shows (and gives) +0.5 in the piety indicator.

The values in the slider indicate change in clergy loyalty of the provinces, not the donations increase.