I'm going to take a wild shot in the dark here and would probably say that it's the cause of increase spawns.
Logic dictates that no objects can occupy the same space in time.
And here is where I'm going with this, IS doesn't create new spawn points, but instead creates more spawns per spawn point. If an enemy is placed too close to a wall or an overhang, the havoc game engine kicks in and the new spawns will sort of bounce and push away from each other as there are multiple spawns appearing on the same point of space. And thus this being a game; while this is happening there will be an occurrence where some spawns will clip through the plane.