Cursor not getting all rows

I'm running:

PostgreSQL 7.4.7 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.2.2
20030222 (Red Hat Linux 3.2.2-5)

I do this:

BEGIN;
SELECT count(*) FROM u, d WHERE u.id = d.id AND ... ;
DECLARE cname CURSOR FOR SELECT u.field, d.field FROM u, d WHERE u.id =
d.id AND ... ;

At the end of the fetching if the number of fetched does not equal the
number from the SELECT count(*) I print out a warning message. It
happens every once in a while. Today it happened four times.

1) missed 1 out of 703773
2) missed 3 out of 703765
3) missed 10 out of 703743
4) missed 12 out of 703660

How is this possible? Because they are in the same transaction the
count and the cursor should see the exact same data, right?