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The packets produced by opus_multistream_encode() when encoding 40ms or 60ms CELT or Hybrid packets for a single stream contain 4 to 6 bytes of padding, which could have been used to add 1 or 2 bytes to each frame.

For example:

$ opusenc --hard-cbr --bitrate 31.5 --framesize 60 comp48s.wav out.opus
Encoding using libopus 1.1-42-ge1f8462 (audio)
-----------------------------------------------------
   Input: 48kHz 2 channels
  Output: 2 channels (2 coupled)
          60ms packets, 31.5kbit/sec CBR
 Preskip: 312

Encoding complete
-----------------------------------------------------
       Encoded: 1 minute and 30.84 seconds
       Runtime: 2 seconds
                (45.42x realtime)
         Wrote: 362224 bytes, 1514 packets, 97 pages
       Bitrate: 31.4667kbit/s (without overhead)
 Instant rates: 31.4667kbit/s to 31.4667kbit/s
                (236 to 236 bytes per packet)
      Overhead: 1.36% (container+metadata)

$

The above command produces 236-byte packets (31.467 kb/s), each containing the 2 bytes required for a code 3 packet, three 76-byte frames, and 6 bytes of padding (including the padding length byte). Instead of the 6 bytes of padding, each frame could have contained 78 bytes of data.

If the multistream API is not used, opus_encode() does produce packets containing three 78-byte frames for this same bitrate and frame size. However, for some reason it also adds 1 byte of padding to each packet, which causes it to slightly exceed the target bitrate.

$ opus_demo audio 48000 2 31500 -cbr -framesize 60 comp48s.pcm out.pcm
libopus 1.1-42-ge1f8462
Encoding 48000 Hz input at 31.500 kb/s in auto mode with 2880-sample frames.
average bitrate:              31.613 kb/s
maximum bitrate:              31.600 kb/s
active bitrate:               31.600 kb/s
bitrate standard deviation:    0.000 kb/s
$