Meaning of vacuum output

I have no idea what the output of
vacuum verbose means.

rupert=# vacuum verbose resp_header;
NOTICE:  --Relation resp_header--
NOTICE:  Pages 28276: Changed 4, Empty 0; Tup 2437108: Vac 0, Keep
2295773, UnUsed 96275.
        Total CPU 1.79s/0.38u sec elapsed 2.72 sec.
VACUUM
rupert=# select count(*) from resp_header;
 count
--------
 141357
(1 row)

There's nowhere near 2.437e6 tuples in the table, but the difference
between Tup and Keep seems to be the number of tuples in the table.  But
what does this mean?  Are there 2.295e6 deleted tuples hanging around in
the table?  That doesn't make sense because vacuum is supposed to toss
deleted tuples (right?).  What's unused?  What is Vac and why is it
*always* 0?  I don't want unused tuples in my tables!

Can you understand my confusion?

Regards,
Jeffrey Baker