Messages are not distributed per machines when consuming from Kafka topic with 1 partition

Messages are consumed from Kafka using KafkaIO. Each kafka topic contains only 1 partition. (That means that messages can be consumed only by one Consumer per 1 consumer group)

When backlog of topic grows and system scales from 1 to X machines, all the messages seems to be executed on  the same machine on which they are read. Due to that message throughput doesn't increase comparing X machines to 1 machine. If one machine was reading 2K messages  per s, X machines will be reading the same amount.