The more mobs are crammed in a small space, the more they will appear to glitch out and float in the air. They are visually not in the crammed space, yet also harmless. They can be right next to me, but they should technically be in my kill chamber of my farm. (This is a similar phenomenon to Minecraft PC (Java) from version 1.2 and below, where in multi-player, they would glitch away from their kill chamber).