Not all kafka consumers are being assigned to partitions

I have 10 consumers and 10 partitions.
I take the number of partitions

    int partitionCount = getPartitionCount(kafkaUrl);

and I create the same number of consumers with the same Group ID.

Stream.iterate(0, i -> i + 1)
.limit(partitionCount)
.forEach(index -> executorService.execute(() -> createCosnumer(consumerConfig(index), topicName)

config looks like this

Properties properties = new Properties();
properties.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, kafkaUrl);
properties.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
properties.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
properties.put(ConsumerConfig.GROUP_ID_CONFIG, CONSUMER_CLIENT_ID);
properties.put(ConsumerConfig.CLIENT_ID_CONFIG, CONSUMER_CLIENT_ID + index);
properties.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "false");
properties.put(ConsumerConfig.SESSION_TIMEOUT_MS_CONFIG, "300000");
properties.put(ConsumerConfig.HEARTBEAT_INTERVAL_MS_CONFIG, "10000");
properties.put(ConsumerConfig.MAX_POLL_INTERVAL_MS_CONFIG, String.valueOf(Integer.MAX_VALUE));
properties.put(ConsumerConfig.PARTITION_ASSIGNMENT_STRATEGY_CONFIG, "org.apache.kafka.clients.consumer.RoundRobinAssignor");

each consumer assigns a topic and begins to consume

consumer.subscribe(topicName);
while (true) {
ConsumerRecords consumerRecords = consumer.poll(Duration.ofSeconds(4));
if (consumerRecords.count() > 0) {
consumeRecords(consumerRecords);
consumer.commitSync();
}
}

what I see after assigning consumers to the partition

TOPIC      PARTITION  CURRENT-OFFSET  LOG-END-OFFSET  LAG             CLIENT-ID                                                        
topicName  1          89391           89391           0               consumer0
topicName  3          88777           88777           0               consumer1
topicName  5          89280           89280           0               consumer2
topicName  4          88776           88776           0               consumer2
topicName  0          4670991         4670991         0               consumer0
topicName  9          23307           89343           66036           consumer4
topicName  7          89610           89610           0               consumer3
topicName  8          88167           88167           0               consumer4
topicName  2          89138           89138           0               consumer1
topicName  6          88967           88967           0               consumer3

only half of consumers have been assigned to partitions
Why did this happened? There must be one consumer per partition according to the documentation. I am doing something wrong? kafka version 2.1.1.