YouTube is currently testing with a small number of Premium members a new video quality option, "1080p Premium", with the subtitle "Enhanced Bitrate". In other words, this option provides 1080p resolution with more data transferred per second, less severe compression, more information per pixel and therefore improved image sharpness.
Pauper's 1080p signal for free users…
After a trial period, the option could soon arrive for all Premium members, and after some concerns were raised, YouTube would like to reassure free users that the "normal" 1080p signal remains accessible to everyone and suddenly won't be restricted to paying subscribers only. The latter will simply benefit from a transfer rate of around 13 MB/s for their 1080p Premium, instead of the usual 8 MB/s. Source: The Verge