I found this sentence somewhat confusing:
"It is possible for a column's value to change even when the trigger
is not fired,"
http://www.postgresql.org/docs/devel/static/sql-createtrigger.html#SQL-CREATETRIGGER-NOTES
More precise would be something along the lines "It is possible that
changes to the
column's value will not cause the trigger to be fired".
The attached patch hopefully rewords the entire paragraph for clarity.
Regards
Ian Barwick