I don't see the picture but I know exactly what you mean.
It means uneven pressure at the bottom of the screen. Unfortunately there's really nothing you can do about that.
You know when you apply too much pressure to a LCD screen display it causes temporary white marks?
E.g bent phone tablet frames are a common cause of this too.
While this is uncommon there are some new devices out there that exhibit this.
Or really it's just called backlight bleed in your case if the surface hasn't been repaired before.
It's a manufacturing imperfection which can appear in some devices.
Update: Here is some info about backlight bleed. It is a manufacturer defect on the screen which I would say is acceptable. Depends on the seller if they will do warranty replacement just because of backlight bleed.