Indexing Function called on VACUUM and sorting ?

It seems functional indexes are recalculated even where it is obviously not
needed.

\d+ test:

i | integer | |
t | text | |
x | text | |
"i_i" btree (i)
"x_iii" btree (xpath_string(x, 'movie/rating'::text))

1)
When I run
VACUUM FULL ANALYZE VERBOSE
OR
VACUUM ANALYZE
a lot of xpath_string calls occur.
Does VACUUM rebuild indexes ? What for to recalculate that all?
It makes VACUUMing very slow.

Simple VACUUM call does not lead to such function calls.

2)
When I do
select * from test order by xpath_string(x, 'movie/rating'::text) limit 1000
offset 10;

Planner uses index x_iii (as it should, ok here): Limit -> Index scan.
But many of calls to xpath_string occur in execution time.
Why ? Index is calculated already and everything is so immutable..

Please answer if you have any ideas.. Functional indexes seemed so great
first, but now I uncover weird issues I can't understand..