Synchronous Replication & replay_location

Hello,

We are currently testing Synchronous Streaming replication between two PG
9.1 boxes () in
a test environment, with the slave as hot_standby.
We appear to have it working correctly (changes appear on both) etc

However the following query from (
http://www.dansketcher.com/2013/01/27/monitoring-postgresql-streaming-replication/
):
SELECT
client_addr,
sent_offset - (replay_offset - (sent_xlog - replay_xlog) * 255 * 16 ^ 6 )
AS byte_lag
FROM (
SELECT
client_addr,
('x' || lpad(split_part(sent_location, '/', 1), 8, '0'))::bit(32)::bigint
AS sent_xlog,
('x' || lpad(split_part(replay_location, '/', 1), 8, '0'))::bit(32)::bigint
AS replay_xlog,
('x' || lpad(split_part(sent_location, '/', 2), 8, '0'))::bit(32)::bigint
AS sent_offset,
('x' || lpad(split_part(replay_location, '/', 2), 8, '0'))::bit(32)::bigint
AS replay_offset
FROM pg_stat_replication
) AS s;

the byte_lag is almost never 0 and usually 816 bytes.

Similiarly, the output from pg_stat_replication *very* rarely has
write_location and replay_location the same.

procpid | usesysid | usename | application_name | client_addr |
client_hostname | client_port | backend_start | state |
sent_location | write_location | flush_location | replay_location |
sync_priority | sync_state
---------+----------+-------------+------------------+-------------+-----------------+-------------+------------------------------+-----------+---------------+----------------+----------------+-----------------+---------------+------------
17895 | 7141183 | replication | walreceiver | 10.2.1.151 |
| 58017 | 2014-04-04 12:41:16.36911+01 | streaming |
17/E6014228 | 17/E6014228 | 17/E6014228 | 17/E6013F08 |
1 | sync
(1 row)

I've been attempting to find documentation about the meaning of
sent_location, write_location, flush_location and replay_location but have
so far been unsuccessful; especially with regards to the implications of
replay_location being < flush_location.

Is this "normal" for synchronous replication?

Many Thanks,
Rob