Varchar Vs. Text index matching - why different?

I just noticed that comparing a text to a varchar results in an index not
being used without a cast in some circumstances. Given that they have
identical internal representations (or so I believe), that seems a little odd.

Can anyone shed some light on this for me? I assume it has something to do
with believing text->varchar is a down-cast due to the size limitation on
varchar, but don't we collect size stats of text fields (stawidth)?

The following illustrates the behaviour:

create table tt(id serial, tf text);
create unique index tfi on tt(tf);
create unique index ttpk on tt(id);

insert into tt(tf) values('fred' || currval('tt_id_seq'));
insert into tt(tf) select 'fred' || currval('tt_id_seq') from tt;

create table vt(id serial, vf varchar(31));
create unique index vfi on vt(vf);
create unique index vtpk on vt(id);

insert into vt(vf) select 'fred' || currval('vt_id_seq') from tt;

analyze;

explain select * from tt,vt where tt.id=12345 and vt.vf = tt.tf;
QUERY PLAN
-----------------------------------------------------------------------------
Nested Loop (cost=100000000.00..100000944.29 rows=1 width=32)
Join Filter: (("inner".vf)::text = "outer".tf)
-> Index Scan using ttpk on tt (cost=0.00..3.01 rows=1 width=16)
Index Cond: (id = 12345)
-> Seq Scan on vt (cost=100000000.00..100000531.68 rows=32768 width=16)
(5 rows)

rt3=# explain select * from tt,vt where tt.id=12345 and vt.vf = cast(tt.tf
as varchar);
QUERY PLAN
----------------------------------------------------------------------
Nested Loop (cost=0.00..7.92 rows=1 width=32)
-> Index Scan using ttpk on tt (cost=0.00..3.01 rows=1 width=16)
Index Cond: (id = 12345)
-> Index Scan using vfi on vt (cost=0.00..4.89 rows=1 width=16)
Index Cond: (vt.vf = ("outer".tf)::character varying)
(5 rows)

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