One thing that the first two levels have in common is that each stage has two enemies: a powerhouse unit/damage sponge paired with several weaker, popcorn-type unit. The mechanic that the player can exploit to breeze through the first level and that the player must exploit to breeze through the second is dependent upon these two units. It makes sense, then, that the third level will also feature two units that react in a similar way.
It being the last non-boss level, it's important that it also be the most hectic-- especially since the second level is really more of a puzzle level. I want to reinforce that this is a shmup and that reflexes count.