autovacuum blocks the operations of other manual vacuum

Hi,

I have a question about the behavior of autovacuum. When I have a big
table A which is being processed by autovacuum, I also manually use
(full) vacuum to clean another table B. Then I found that I always got
something like “found 0 removable, 14283 nonremovable row”. However,
if I stop the autovacuum functionality and use vacuum on that big
table A manually, I can clean table B (ex. found 22615 removable, 2049
nonremovable row).

Is this correct? Why do vacuum and autovacuum have different actions?

Ps. My postgreSQL is 8.4.