This issue was discussed in the Jan 17, 2018 Support meeting.

How messages are distributed to consumers isn't entirely intuitive. Using the default buffer settings (consumerWindowSize?), messages are distributed in the most efficient manner, which means they're essentially delivered to consumers in batches that get the stack to zero as fast as possible. For example, with the default settings, if one had 100 messages and 5 consumers, each consumer would get 20 messages in turn

One needs to "dial down" the buffer (can be done on broker or clients) to achieve true round-robin distribution. With 100 messages and 5 consumers, each consumer would get one message each until the stack is zero.

(Justin, I know you know this all. Just trying to get it to stick in my brain. )