Sorry, discard my previous reply. I did not read that you were updating from the results window.
While updating from the results window, only fields from a table which are present in the result set, are used for updating.
Lets say you have a table with primary key 'id' and another field 'data'.
If your result set has only 'data' in it, then it will update the table with the query
UPDATE table SET field = value2 WHERE field = value1
as other fields in the table are not available.
If the table's primary key is part of the result set, then it will use the primary key in the where clause.