AMOLED screens can turn pixels off if the color they're displaying is true black (#000000).
This allows them to save battery and display a truer black than LCDs are capable of.
If the color isn't true black, then the screen won't turn the pixel off.
Sometimes, for whatever reason, artists (or compression algorithms) don't actually use true black for the background, instead it'll be something like #0f0f0f or #030303.
On a LCD these colors are indistinguishable from true black, but it is noticeable on a AMOLED screen.