pg_class.relpages not updated for toast index

Was doing a relation size estimation based on pg_class.relpages of the relation and the related objects (index, toast) and noticed that it is not updated for the toast index, for example:

fabrizio=# CREATE TABLE t(c TEXT);
INSERT INTO t VALUES (repeat('x', (8192^2)::int));

VACUUM (ANALYZE) t;
CREATE TABLE
INSERT 0 1
VACUUM
fabrizio=# \x on
Expanded display is on.
fabrizio=# SELECT
  c.oid,
  c.relname,
  c.relpages,
  t.relname,
  t.relpages AS toast_pages,
  ci.relname,
  ci.relpages AS toast_index_pages,
  (pg_stat_file(pg_relation_filepath(ci.oid))).size AS toast_index_size
FROM
  pg_class c
  JOIN pg_class t ON t.oid = c.reltoastrelid
  JOIN pg_index i ON i.indrelid = t.oid
  JOIN pg_class ci ON ci.oid = i.indexrelid
WHERE
  c.oid = 't'::regclass;
-[ RECORD 1 ]-----+---------------------
oid               | 17787
relname           | t
relpages          | 1
relname           | pg_toast_17787
toast_pages       | 97
relname           | pg_toast_17787_index
toast_index_pages | 1
toast_index_size  | 16384
--