type cast about int to bit

hi，
there is a problem about type cast that i don't understand, follow is my test.

postgres=# select 10::bit(3);
bit
-----
010
(1 row)
postgres=# select 10::bit varying(3);
ERROR: cannot cast type integer to bit varying
LINE 1: select 10::bit varying(3);
^
postgres=#

my question is why int can cast to bit , i want to know the reason.
thank you for your timing.