reading vacuum verbose output

Hello list!
I have run a vacuum verbose on a db and got something a can't understand
by myself :

INFO:  Pages 4768: Changed 0, Empty 0; Tup 4764: Vac 0, Keep 0, UnUsed 542404.
        Total CPU 0.35s/0.00u sec elapsed 1.51 sec.
INFO:  --Relation public.student2maslulsign_rashum_musmach_old--
INFO:  Pages 906: Changed 0, Empty 0; Tup 846: Vac 0, Keep 0, UnUsed 96035.
        Total CPU 0.03s/0.01u sec elapsed 0.04 sec.
INFO:  --Relation public.preconditioncourse--
INFO:  Pages 297: Changed 0, Empty 0; Tup 3754: Vac 0, Keep 0, UnUsed 24163.
        Total CPU 0.01s/0.00u sec elapsed 0.02 sec.
INFO:  --Relation public.course--
INFO:  Pages 40891: Changed 0, Empty 0; Tup 13203: Vac 0, Keep 0, UnUsed 1413836.
        Total CPU 1.22s/0.17u sec elapsed 2.94 sec.
INFO:  --Relation public.counter--
INFO:  Pages 6050: Changed 0, Empty 0; Tup 10701: Vac 0, Keep 0, UnUsed 690548.
        Total CPU 0.28s/0.01u sec elapsed 5.91 sec.

If there are unused tuples why i see the Changed 0 ?
Probably i misunderstand the output fields can somebody please explain
those to me?
(Note :The db is vacuumed every ~5 hours ver 7.3)
Thanks.