My theory on this is that Windows 8 hyped this. It's particularly problematic when it's being copied in mac circles because OSX has a biased font-smoothing alg

My theory on this is that Windows 8 hyped this. It's particularly problematic when it's being copied in mac circles because OSX has a biased font-smoothing algorithm that adds font weight (it's a design flaw). In other words, to get something to look thin on a mac, especially a non-retina mac, it needs to be very thin, sometimes less than a pixel thin. How other systems display borderline visible strokes varies depending on the system and the details of the font.