sox gives inconsistent results mixing stereo to mono

Whichever of the following ways of mixing stereo to mono I try *repeatedly*, the resulting files are always different (md5 hashes do not match):

sox stereo.wav -c 1 mono.wav
sox stereo.wav mono.wav remix 1,2
sox stereo.wav mono.wav remix 1-2

The differences appear to be in the binary meat of the audio, not in the headers. If I use say ffmpeg, the resulting file is always the same:

ffmpeg -hide_banner -i stereo.wav -ac 1 mono.wav

Does sox use some sort of randomness in the mixing-down algorithm? Why?