The unpushed button (top) has a dark bottom edge. Sun don’t shine there, son.
The unpushed button is slightly brighter at the top than at the bottom. This is because it imitates a slightly curved surface. Just as how you’d need to tilt a mirror held in front of you up to see the sun in it, surfaces that are tilted up reflect a biiiiit more of the sun’s light towards you.
The unpushed button casts a subtle shadow– perhaps easier to see in the magnified section.
The pushed button, while still darker at the bottom than at the top, is overall darker– this is because it’s at the plane of the screen and the sun can’t hit it as easily. One could argue that all the pushed buttons we see in real life are darker too, because our hands are blocking the light.