audio – What are the two bitrates in the output of `ffprobe`?

I have an audio file and if I run ffprobe on it, I get:

  Duration: 00:03:11.17, start: 0.000000, bitrate: 129 kb/s
    Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 128 kb/s (default)

Note the two bitrate values: I would like to know why there are two and what they are?

Here they are close but not identical. In other audio files, the two are identical. In other cases they are widely different:

Input #0, ogg, from 'Oxygen-Sys-Warning.ogg':
  Duration: 00:00:02.07, start: 0.000000, bitrate: 159 kb/s
    Stream #0:0: Audio: vorbis, 48000 Hz, stereo, fltp, 192 kb/s
Input #0, ogg, from 'Oxygen-Window-All-Desktops-Not.ogg':
  Duration: 00:00:00.60, start: 0.000000, bitrate: 112 kb/s
    Stream #0:0: Audio: vorbis, 44100 Hz, mono, fltp, 96 kb/s

If I want to run a filter on an audio file, I want to output a file with identical sampling rate and bitrate, so I want to know which is the correct bitrate and why two are displayed.