JavaFX 2.0 :: TableColumn CellValueFactory Invoked For Every Cell Update - Normal?

I have a JavaFX 8 TableView with multiple columns.  The CellValueFactories for the columns are set as:
 
xxxColumn.setCellValueFactory(new ItemCountFactory(itemName));  

where itemName is a String identifying a different item for each column.
 
ItemCountFactory returns a StringBinding as the cell value, as follows:
 
    public ObservableValue<String> call(CellDataFeatures<Category, String> rowData) {
        return new ItemCountBinding(rowData.getValue(), itemName);
    }
 
where
     class ItemCountBinding extends StringBinding {
     ...
          protected String computeValue() {
                    String cellValue = ...;
                    return cellValue;
          }
     }

The StringBinding depends on properties of the Category instance for the table row.  The correct data is displayed okay, but I have noticed that every time one of the Category properties changes (i.e., every time a cell value changes), ItemCountFactory is called again to create a new StringBinding.  I would have expected the Factory to be invoked just once for each cell during initialization, and then the table column would monitor the returned Observable object.  Instead it seems to be creating a new Observable object every time the cell value changes.  Is this normal, or am I doing something wrong?  Same behavior on Java 8u20 and 8u40.  If it is not the expected behavior, I will write an example test program; I do not want to post the current full source code.
 
The table items are set just once, with:

     countTable.setItems(categoryList);

where categoryList is of type ObservableList<Category>.  The list itself is not modified after being associated with the table; i.e., no rows are added or deleted or replaced.