This is from the api docs ComputeCentroid Method which is what the node is likely using to calculate the value from the solid
Calculates the centroid of the solid using an approximation, with an accuracy suitable for architectural purposes. This will correspond only with the center of gravity if the solid represents a homogeneous structure of a single material.
The solution here maybe to find the mean location of all the solids’ centroids that compose the family rather than perform the union and take the centroid
Alternately take the bounding box of the geometry and get the average of the max and min, however that is likely to be less accurate