Unpredicatable behavior of volatile functions used in cursors

Hi,

I'm running into some inconsistent behavior when using volatile functions with cursors under PG 8.1.

We're using the following technique for counting the number of rows in a cursor:

-----------
DECLARE instance_cur_1 SCROLL CURSOR FOR
SELECT util.row_number(), *
FROM (
    $LONG_RUNNING_QUERY
) ss

FETCH LAST IN instance_cur_1;
-----------

util.row_number() is a volatile function written in C which simply returns "++internal_counter" every time it is
called.

What's unusual is that for some queries FETCH LAST returns a row_number value which matches the actual number of rows,
while for others it returns the actual number +1 (and adds +1 for each consecutive call). It seems that under some
conditions util.row_number() gets re-evaluated for every call.

Could someone explain why and under which conditions this happens?
Is there a way to make this behavior more consistent?

Regards,
Aleksander