pg_last_xact_replay_timestamp lies

Hello, dear guru =) help me to fix situation when no change made on primary and standby show lag more than 15 minutes:
master :
postgres=# select * from pg_stat_replication
;
-[ RECORD 1 ]----+------------------------------
pid              | 18553
usesysid         | 117942
usename          | replication
application_name | walreceiver
client_addr      | 10.62.43.60
client_hostname  |
client_port      | 45281
backend_start    | 2015-06-15 00:02:00.095195+03
state            | streaming
sent_location    | 62/BC000000
write_location   | 62/BC000000
flush_location   | 62/BC000000
replay_location  | 62/BC000000
sync_priority    | 0
sync_state       | async

standby:
postgres=# SELECT (extract(epoch from now() - pg_last_xact_replay_timestamp() ))::int AS slave_lag;
 slave_lag
-----------
      1030

on master :
postgres=# select name,setting from pg_settings where lower (category) like '%write%log%';
             name             |                              setting                             
------------------------------+-------------------------------------------------------------------
 archive_command              | cp -i %p /var/lib/postgresql/9.3/main/pg_archivelog/%f </dev/null
 archive_mode                 | on
 archive_timeout              | 600
 checkpoint_completion_target | 0.7
 checkpoint_segments          | 32
 checkpoint_timeout           | 300
 checkpoint_warning           | 30
 commit_delay                 | 0
 commit_siblings              | 5
 fsync                        | on
 full_page_writes             | on
 synchronous_commit           | on
 wal_buffers                  | 2048
 wal_level                    | hot_standby
 wal_sync_method              | fdatasync
 wal_writer_delay             | 200
(16 rows)