FYI, I read about this issue so when I chose my case in the store, I asked them if I could open it up there to make sure it was good. I did, and it was pretty much perfect. So then I went home, put it on my phone, and now 4 days later the corners have developed small creases in them just like your picture.
I think just the act of holding the phone will cause that in time. The fit isn't very tight when you first put it on the phone, so after holding and gripping it for a while, the corners curl in a little more, which causes those bends. Obviously some of the cases developed that just from being packaged. Either way, it seems natural for leather, and you only will notice when you look really closely.
Don't worry about it. The case still looks really nice!