ComboBox and set_cell_data_func

[Jeffrey Barish <jeff_barish earthlink net>]

I am using set_cell_data_func with a ComboBox.  I noticed that when I call
the append method of the TreeStore, set_cell_data_func is called twice. 
The first time, the row has not yet been written into the TreeStore, so the
set_cell_data_func can't perform its function.  Is there a reason that GTK
calls set_cell_data_func twice?
-- 
Jeffrey Barish