Wrong number of empty toast pages

Hi,

I have a problem with growing toast table in pgsql 8.3.5.  Probably
raising max_fsm_pages and running autovacuum more aggresively will fix
it, but during investigation I've got this puzzling output from VACUUM
ANALYZE VERBOSE:
> INFO:  "pg_toast_16505": found 756385 removable, 23003704 nonremovable row versions in 28446864 pages
> DETAIL:  14586 dead row versions cannot be removed yet.
> There were 92404502 unused item pointers.
> 22769942 pages contain useful free space.
> 42 pages are entirely empty.

So there are 23M rows in 28.5M pages.  Assuming worst case of 1 row per
page, there should be 5.5M completely free pages.  But vacuum says that
only 42 are entriely empty.   Am I missing something obvious here?

Best regards,
Jacek.