join/from_collapse_limit and geqo_threshold default values

I wonder why join_collapse_limit default values is set to 8 but
geqo_threshold is 12. Optimizer doesn't change the order of JOIN's of
queries that contains from 8 to 11 tables. Why it's 'wise' decision as
documentation says?

from_collapse_limit (integer)

The planner will merge sub-queries into upper queries if the
resulting FROM list would have no more than this many items. Smaller
values reduce planning time but might yield inferior query plans.
The default is eight. It is usually wise to keep this less than
geqo_threshold
<http://www.postgresql.org/docs/8.3/static/runtime-config-query.html#GUC-GEQO-THRESHOLD>.
For more information see Section 14.3
<http://www.postgresql.org/docs/8.3/static/explicit-joins.html>.