Why does the node value not exactly match the mean of the data records contained in a node?
The map is a representation of the data records that smooth out effects like noise and outliers. The node values are responsible to determine which data records are matched into a respective node. This does not necessarily mean that an attribute mean of all records falling into some node is the same as the node attribute value. This is only approximately the case and can be violated particularly in the presence of outliers.
