iPhone 6s Space Gray cover backward question

Hello. I have a small question to you regarding your iPhone 6s Space Gray. I mean specifically the rear of the casing and, specifically, the plastic inserts at the back of the phone (specifically those at the bottom). In the picture I marked the place. I would like to know if this place is there to combine plastic visible at the right angle depends on how you look (I have attached the closer it will be can see) .u the top of this there is, and I wonder if it should be. The phone never fell etc. I wonder if it's some sort of crack or defect of the material. On top sincerely thanks for the reply

regards Krzysztof

 

Someone from the holders of 6s Space Gray is unable to confirm whether such joining of plastic in them occurs or not ?