Sorting on longer key is faster ?

The 2 queries are almost same, but ORDER BY x||t is FASTER than ORDER BY x..

How can that be possible?

Btw: x and x||t are same ordered

phoeniks=> explain analyze SELECT * FROM test WHERE i<20 ORDER BY x || t;
QUERY PLAN

----------------------------------------------------------------------------
----------------------------------------------
Sort (cost=2282.65..2284.92 rows=907 width=946) (actual
time=74.982..79.114 rows=950 loops=1)
Sort Key: (x || t)
-> Index Scan using i_i on test (cost=0.00..2238.09 rows=907 width=946)
(actual time=0.077..51.015 rows=950 loops=1)
Index Cond: (i < 20)
Total runtime: 85.944 ms
(5 rows)

phoeniks=> explain analyze SELECT * FROM test WHERE i<20 ORDER BY x;
QUERY PLAN
----------------------------------------------------------------------------
---------------------------------------------
Sort (cost=2280.38..2282.65 rows=907 width=946) (actual
time=175.431..179.239 rows=950 loops=1)
Sort Key: x
-> Index Scan using i_i on test (cost=0.00..2235.82 rows=907 width=946)
(actual time=0.024..5.378 rows=950 loops=1)
Index Cond: (i < 20)
Total runtime: 183.317 ms
(5 rows)

phoeniks=> \d+ test
Table "public.test"
Column | Type | Modifiers | Description
--------+---------+-----------+-------------
i | integer | |
t | text | |
x | text | |
Indexes:
"i_i" btree (i)
"x_i" btree (xpath_string(x, 'data'::text))
"x_ii" btree (xpath_string(x, 'movie/characters/character'::text))
Has OIDs: no