Understanding pg_last_xlog_receive_location

I'm following the documentation here (using postgresql 9.4.5):
https://www.postgresql.org/docs/9.4/static/functions-admin.html

I'm attempting to fully understand the interplay
between pg_is_in_recovery() + pg_last_xlog_receive_location() +
pg_last_xlog_replay_location() so we can devise a reliable health check
script.

Here's a database that is configured as a hot standby for streaming
replication.

appdb=> SELECT pg_is_in_recovery(), pg_last_xlog_receive_location(),
pg_last_xlog_replay_location(); pg_is_in_recovery |
pg_last_xlog_receive_location | pg_last_xlog_replay_location
-------------------+-------------------------------+------------------------------
t | | 0/70A4C88 (1 row)

Note that the DB is in recovery, but that pg_last_xlog_receive_location is
NULL while pg_last_xlog_replay_location is 0/70A4C88.

I'm not sure I understand how this is possible. According to the docs,
pg_last_xlog_receive_location can only be NULL when streaming is disabled
(not the case) or hasn't started yet (doesn't seem possible when
pg_last_xlog_replay_location is set).

Could someone help shed some light on what state results in
pg_last_xlog_receive_location being NULL when pg_last_xlog_replay_location
is set?