postgres serialiazation errors where none should happen (false negatives)

sqlest.sh

#!/bin/bash

DBNAME=dbname
DBUSER=dbuser
N=150

sql () {
    echo $1 | psql -h localhost $DBNAME $DBUSER
}

sql "drop table if exists test;create table test (key int, num int);" > /dev/null

STMT=""
for i in $(seq $N); do
    STMT="$STMT insert into test(key, num) values($i, 0);"
done
sql "$STMT" > /dev/null

echo
echo "setup done"
for i in $(seq $N); do
    sql "begin isolation level serializable; update test set num=1 where key=$i; commit;" > /dev/null &
done
echo "waiting"
wait
sql "select count(*) from test where num=0"