Stained glass seems brighter when facing Z axis

I built a wall of barrier block from y=0 to y=255, fencing 3*3 chunks(at about (1000,0,0)), and a white stained glass wall covering it. When i looked outside, the stained glass wall seemed brighter when i'm facing z±, than x direction. 

World settings: super flat: the void

My video settings: 

Graphics: Fancy; Smooth Lighting: max; VSync: On; View Bobbing: on; Brightness: Bright;