I think it may also depend on the subpixel geometry[0][1] (and pixel density) of the screen. There are a lot of ways to arrange subpixels.

[0] https://en.wikipedia.org/wiki/Pixel_geometry

[1] https://en.wikipedia.org/wiki/PenTile_matrix_family

Is it because you're viewing it at a different distance (phone usually closer than monitor) and therefore the focusing variation is different?

Worked for me on my iPhone 6s+, though not as strong as OP described. Maybe LCD vs OLED?