Weird happenings inside the package

I created package BOB as :

create or replace package bob as
  procedure getcount(v_tabname in varchar2); end;
/

create or replace package body bob is
procedure getcount(v_tabname in varchar2) as   h_count number;
  begin
    select count(*) into h_count
      from all_tables where table_name=UPPER(v_tabname);     dbms_output.put_line(h_count);
  end
end bob;
/

create public synonym bob for bob;
grant execute on bob to public;

If I execute the package using the login that created it (SYSTEM) then it returns the correct value. If, however, I connect as another user (that does have select rights on ALL_TABLES) the procedure returns zero, but running the select statement that sets h_count returns the correct value. Why ?

--
Peasant