It's probably the OLED grain as mentioned above. It's very noticeable on mine—not sure if I got a dud or if I'm just sensitive to it, but the grain is obvious even on lighter/non-grey backgrounds. For text/fine-detail, it shows as slight color fringing on borders which makes it look soft/fuzzy. It normally doesn't bother me but there are certain situations where it's obvious and off-putting.
Assuming your pictures are accurate, just look at the solid lines above and below the "Sign In" text. On Mac, it's a uniform color. On iPad, it's noticeably nonuniform and you can pick out different colors.