Why is sample rate optional only for FLAC or WAV file and not other formats?

[bruno_medeiros]

So for example at my work we are using WEBM_OPUS encoding, which from what I understand, specificies the sample rate in audio stream metadata itself? Yet from here: https://cloud.google.com/speech-to-text/docs/basics#sample-rates it says the field is only optional  for FLAC or WAV formats.

And indeed, when I try the GSTT API with some example code (Streaming Recognition and a WEBM_OPUS encoded at 48000 sample rate), the GSTT actually accepts sample rates other than 48000 - and depending on the recognition model, produces different results depending on the sample rate selected!