My personal theory is that stuff looks / renders differently on Macs due to them having 1) retina screens, 2) different font smoothing algorithms, 3) glossy screens.
Most designers work exclusively on Macs and they are not even aware of the issue.
In particular, very thin fonts look well on Macs while they look like absolute garbage under Windows (to the point where some serifs are plainly not displayed so you can't recognize the letters).
I also think that certain "gray on gray" combinations on average have higher contrast on Mac displays than on non-Macs.
Most designers work exclusively on Macs and they are not even aware of the issue.
In particular, very thin fonts look well on Macs while they look like absolute garbage under Windows (to the point where some serifs are plainly not displayed so you can't recognize the letters).
I also think that certain "gray on gray" combinations on average have higher contrast on Mac displays than on non-Macs.