Sooo.. lately i have noticed that I see some regular old 2d images with depth.. It seems to primarily happen with images that have bright blue in them. For example, this images by snake85027 in the pixel art topic. The Blue BG feels like it has depth to it, making the tree feel like it pops out. Or this one, The blue BG…
Your brain is working fine ! You might want to look up "atmospheric perspective" :) These UIs overlays having stronger value contrast than the games behind them are great examples of this. Now of course in these cases, who knows if it was intentional in the first place ! But I agree, it really works.