jsonb on-disk size calculation

hi.

drop table x;
create table x(js jsonb);
insert into x select '{"Hello world":1}'::jsonb;
select pg_column_size(js) from x; -- return 33.

based on src/include/utils/jsonb.h
The key and value part is 20 bytes (is it correct?), Jsonb->vl_len_ is 4
byte, JsonbContainer->header is 4 bytes. That's 28 bytes.

but now on-disk is 33 bytes.
so I am not sure where the remaining bytes are.