VACUUM and transaction ID wraparound

The documentation says (Admin Guide, 8.2.3), "... each database-wide VACUUM automatically delivers a warning" if
individualtables need VACUUMing to prevent transaction ID wraparound. 

But I thought a database-wide VACUUM automatically VACUUMed each table in the database (Ref Manual, 'VACUUM'), making
(atleast to my way of thinking) VACUUMing an individual table unnecessary, and thus making the warning superfluous. 

Is there something here I don't understand?

-- Dean