It's simply a way to break up the large amount of vocabulary and patterns to learn. So, due to the number of skills covered and the amount of vocabulary that you'll encounter at each level, Glossika gives you exposure to as large a variety of sentence structures with smaller sets of data, by splitting each level into high and low. Otherwise, you may find yourself stuck doing one kind of skill for a very long time and it may lead to boredom or less satisfaction. The goal is to keep you engaged and keep making progress without feeling completely overwhelmed by an endless number of similar sentences.
All of the skills are covered in each level, except the first, as there are some complex sentences that you shouldn't be exposed to in A1-Low so as not to overwhelm the beginner.