ffmpeg with h264 produces smaller file sizes for veryfast preset

Question

I am experimenting with ffmpeg and x264 codec. And I have found some unexpected behaviour.

I am encoding video with parameters CRF 23, high profile and changing preset. Documentation says:

for constant quality encoding, you will simply save bitrate by choosing a slower preset.

And I am using CRF, so it should be this particular quote. So how is possible that I have achieved the smallest file for veryfast preset?

medium    256MB
fast      261MB
faster    235MB
veryfast  209MB
superfast 324MB

What am I missing? BTW another comparison have similar results.

Answer 1

I was able to find an answer. https://stackoverflow.com/a/14308561/941493.

In the nutshell, CRF calculation is also affected by preset so the same CRF number is not the same with different presets. Based on some developer difference can be mostly ignored.