another wal archive question

When reading the docs on recovery.conf, I noticed this:

WAL segments that cannot be found in the archive will be sought in pg_xlog/;
this allows use of recent un-archived segments. However segments that are
available from the archive will be used in preference to files in pg_xlog/.
The system will not overwrite the existing contents of pg_xlog/ when
retrieving archived files.

If the system will use the files in the archive in preference to the ones in
pg_xlog, how can this actually happen if it will not overwrite the contents of
pg_xlog?

--
Jeff Frost, Owner <jeff(at)frostconsultingllc(dot)com>
Frost Consulting, LLC http://www.frostconsultingllc.com/
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