rule which unpredictable modify a sequence number

I have a problem with a rule which unpredictable modify a sequence
number. When I add a new table entry, the new id_keyword hasnt the value
as expected. ex.: 1000000000000000, 1000000000000001,
1000000000000002,...... If i remove the rule it works.

Here the table, rule and sequence definitions:

CREATE TABLE geocontexter.gc_keyword(
   id_keyword bigint NOT NULL DEFAULT nextval('geocontexter.seq_gc_keyword'::regclass),
   id_parent bigint NOT NULL DEFAULT 0,
   id_attribute_group bigint,
   id_status smallint NOT NULL DEFAULT 100,
   update_time timestamp without time zone NOT NULL,
   preferred_order smallint,
   lang character varying(30) NOT NULL DEFAULT 'en'::character varying,
   title character varying(126) NOT NULL,
   description character varying(10000) NOT NULL DEFAULT ''::character varying,
   attribute_value text,
   CONSTRAINT gc_keyword_id_keyword PRIMARY KEY (id_keyword)
)
WITH (
   OIDS=FALSE
);

CREATE OR REPLACE RULE keyword_insert_or_replace AS
     ON INSERT TO geocontexter.gc_keyword
    WHERE (EXISTS ( SELECT 1 FROM geocontexter.gc_keyword
                    WHERE gc_keyword.id_keyword = new.id_keyword))
    DO INSTEAD
           UPDATE geocontexter.gc_keyword SET lang = new.lang, description = new.description,
                                              title = new.title, update_time = new.update_time, id_parent =
new.id_parent,
                                              preferred_order = new.preferred_order, id_attribute_group =
new.id_attribute_group,
                                              attribute_value = new.attribute_value
           WHERE gc_keyword.id_keyword = new.id_keyword AND gc_keyword.update_time<  new.update_time;

CREATE SEQUENCE geocontexter.seq_gc_keyword
   INCREMENT 1
   MINVALUE -1999999999999999
   MAXVALUE 1999999999999999
   START           1000000000000000
   CACHE 1
   CYCLE;