With OLED the black levels are affected by random pixel mura—which makes pixels that should be off show as slightly on. So you can cover that by biasing them up

With OLED the black levels are affected by random pixel mura—which makes pixels that should be off show as slightly on. So you can cover that by biasing them up slightly so they are all slightly on and the blacks look more uniform—however the actual contrast ratio suffers. The higher the PPI for oled typically the worse the mura issue gets. So likely they will improve this over time to get the black levels down and effectively the contrast ratio will go way up.