MP3 encoding bitrates go above 320Kbps

[Paulo Fidalgo]

Summary of the bug: When I encode an mp3 using the guide in documentation, I'll get frames with bitrate higher than 320Kbps.
How to reproduce:

% ffmpeg -i 2L38_01_96kHz.flac -codec:a libmp3lame -qscale:a 0 2L38_01_96kHz.mp3

The file used is from here:
​http://www.lindberg.no/hires/test/2L38_01_96kHz.flac

I'm using ffmpeg from rpm-fusion in Fedora 20:

 ffmpeg version 2.1.5 Copyright (c) 2000-2014 the FFmpeg developers
  built on Jul  8 2014 20:44:17 with gcc 4.8.3 (GCC) 20140624 (Red Hat 4.8.3-1)
  configuration: --prefix=/usr --bindir=/usr/bin --datadir=/usr/share/ffmpeg --incdir=/usr/include/ffmpeg --libdir=/usr/lib64 --mandir=/usr/share/man --arch=x86_64 --optflags='-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic' --enable-bzlib --disable-crystalhd --enable-frei0r --enable-gnutls --enable-libass --enable-libcdio --enable-libcelt --enable-libdc1394 --disable-indev=jack --enable-libfreetype --enable-libgsm --enable-libmp3lame --enable-openal --enable-libopencv --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libv4l2 --enable-libvpx --enable-libx264 --enable-libxvid --enable-x11grab --enable-avfilter --enable-avresample --enable-postproc --enable-pthreads --disable-static --enable-shared --enable-gpl --disable-debug --disable-stripping --shlibdir=/usr/lib64 --enable-runtime-cpudetect
  libavutil      52. 48.101 / 52. 48.101
  libavcodec     55. 39.101 / 55. 39.101
  libavformat    55. 19.104 / 55. 19.104
  libavdevice    55.  5.100 / 55.  5.100
  libavfilter     3. 90.100 /  3. 90.100
  libavresample   1.  1.  0 /  1.  1.  0
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
Hyper fast Audio and Video encoder 

For reference there is a note in wikipedia in bitrate section:
​http://en.wikipedia.org/wiki/MP3

Non-standard bit rates up to 640 kbit/s can be achieved with the LAME encoder and the freeformat option, although few MP3 players can play those files. According to the ISO standard, decoders are only required to be able to decode streams up to 320 kbit/s

[Carl Eugen Hoyos]

I believe this was very well explained on the mailing list:
​http://thread.gmane.org/gmane.comp.video.ffmpeg.user/54373/focus=54374