It's more like SDR goes from 0 to 255 and HDR goes from 0 to 1024. In SDR mode, 255 = (say) 500 nits while in HDR mode 1024 = 1000 nits and thus 255 = 250 nits

It's more like SDR goes from 0 to 255 and HDR goes from 0 to 1024. In SDR mode, 255 = (say) 500 nits while in HDR mode 1024 = 1000 nits and thus 255 = 250 nits so SDR content looks dimmer.