H.265(libx265) codec for 8K videos, using same compress settings, why yuv444 10-bit is saving bitrates than yuv420 8-bit? Our workflow involves huge 8K clips(i.e., 8192x8192). The inputs are mostly very high bitrates 264/265 clips, though we are using ProRes or  DNxHD for post-production within the process, we do not want to archive such huge files as the "master" copies(easily over 100GB for 1 minute).

My current approach is, using FFMPEG to compress the ProRes/DHxHD clips into H.265 clips using a very high-quality setting (crf=12, slow profile).

Another quality related setting is the pixel format config. Then my question comes up:

Targeting the same clips(5 of them), using the same compress settings, one with yuv420 8-bit, one with yuv444 10-bit, yuv444 outputs are constantly smaller than yuv420 outputs(5%-10%), i.e, lower bitrates.

If yuv444 10-bit needs to store more information, how could this be possible?
From Why does 10-bit save bandwidth (even when content is 8-bit)?


  When encoding with the 10-bit tool, the compression process is
  performed with at least 10-bit accuracy compared to only 8-bit
  otherwise. So there is less truncation errors, especially in the
  motion compensation  stage, increasing the efficiency of compression 
  tools. As a consequence, there is less need to quantize to achieve a
  given bit-rate.
  
  The net result is a better quality for the same bit-rate or conversely
  less bit-rate for the same quality: between 5% and 20% on typical
  sources.


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