Comment by arch1t3cht

libav{format,codec,...} are just libraries for demuxing and decoding video. There is huge variability in how those libraries are used, let alone how the video is displayed (which needs scaling, color space conversions, tonemapping, subtitle rendering, handling playback timing, etc. etc.). mpv also has its own demuxer for matroska files, since libavformat's is very limited [1].

[1] https://github.com/mpv-player/mpv/wiki/libavformat-mkv-check...