You referrin to the front being bright enough but the back being dark? It's probably either because :-

1.The "back" ur referrin to is pretty darn far,i.e you're flash doesn't reach that distance.Despite having a xenon flash,being a cameraphone,it does lose out to regular cameras' flash.

2.The other reason would be the other way around,i.e. the "front" you refer to is pretty close up.Assuming you focus on this "front" object,the camera calculates the distance and reduces the power of the flash,as not to over-expose ur main focus object (so it doesn't look overly bright)

Cropped the picture? Nice perspective result