[Gvsig_english] The two functions x() and y() and polygon themes: what is calculated?

[Wolfgang Qual]

Hi list,
I need to add the x- and y-coordinates of the centroids of a polygon theme. 
First, I thought that this was only possible by creating a centroid point 
theme first (with sextante) and then joining that point layer with the source 
theme (polygon) using the spatial join function of the geoprocessing wizard.
By accident, I found out that it is also possible to apply the field 
calculator's functions x() and y() to polygon shapefiles. Very nice, I 
thought. However, when comparing the location of the centroids with the 
calculated x- and y-coordinates of the polygon theme, I noted that both 
coordinates are not identical. Therefore, I would like to ask you all: what 
is (or should be) calculated when x() and y() is applied to polygon themes? 

Best,
Wolfgang