They are random - this would be great feedback for the product dev team to be able to chang the order. I believe it's done this way since the amount of time spent on a certain screen is short - randomization allows each banner to get a chance to be seen.Â
For example: if you are a speedy app user, you may never stay on any given screen for more than 5 seconds at any given time. The banners that appear after 6 seconds will never be seen.