This is a behavior you can observe with just a 2-2 splitter. Splitters only care about the amount of items going through them. As long as the amount of items are correct, the splitter doesn't guarantee that it'll produce any particular item mixing pattern. Balancers are the same, since they're built on splitters. To mix items you'll need have a sushi belt setup of some sort. The splitter versions of such setups are able to guarantee item mixture because they only use 1-2 and 2-1 splitters.