[CFD - Post] Discrepancy between areaInt of Pressure
 

I have done a transient CFD calculation on the geometry shown in the attachment. The flow is in the negative z-direction.
With the Function Calculator I now try to determine the forces on the triangular faces individually and together for the full geometry.
However, when I do an area integral over one triangular surface in the y-direction, then do the same to the other triangular surface and sum the result I get a lower value (around 5% lower) than when I do the area integral over the two faces together.

Why is this the case?