why ffmpeg encodes a ogg file with flac encoder?

I want to convert an audio into an ogg file, my command is:

ffmpeg -i in.wav out.ogg

I found that the output stream encoder is flac instead of ogg or vorbis !

The ffmpeg output message shows below:

ffmpeg version 3.1.2 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.9.2 (GCC)
Guessed Channel Layout for Input Stream #0.0 : mono
Input #0, wav, from 'in.wav':
  Metadata:
    encoder         : Lavf55.48.100
  Duration: 00:00:11.97, bitrate: 256 kb/s
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 16000 Hz, 1 channels, s16, 256 kb/s
[ogg @ 0x8003e020] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead.
Output #0, ogg, to 'out.ogg':
  Metadata:
    encoder         : Lavf57.41.100
    Stream #0:0: Audio: flac, 16000 Hz, mono, s16, 128 kb/s
    Metadata:
      encoder         : Lavc57.48.101 flac
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le (native) -> flac (native))

Anyone tell me why ? Thanks !

Solution

You need a ffmpeg build with libvorbis to encode to Vorbis. As per the banner of your build, it's not present, so it uses Flac, a native encoder, supported in Ogg.

Get a full-featured binary for your OS from here.