AVI -> H263 -> AVI = Duration 10sec -> Duration 10sec -> Duration 6sec???

Hi there,
 
I call FFMPEG as follows:
 
bash# ffmpeg -i foreman.avi -f rawvideo -vcodec h263p
foreman_raw_converted.rtp                   FFmpeg version CVS,
Copyright (c) 2000-2004 Fabrice Bellard
  configuration:  --enable-shared --enable-static
  libavutil version: 49.0.0
  libavcodec version: 51.4.0
  libavformat version: 50.2.1
  built on Mar 27 2006 12:28:53, gcc: 3.3.3
Input #0, avi, from 'foreman.avi':
  Duration: 00:00:10.0, start: 0.000000, bitrate: 9128 kb/s
  Stream #0.0, 15.00 fps(r): Video: rawvideo, bgr24, 176x144
File 'foreman_raw_converted.rtp' already exists. Overwrite ? [y/N] y
Output #0, rawvideo, to 'foreman_raw_converted.rtp':
  Stream #0.0, 15.00 fps(c): Video: h263p, yuv420p, 176x144, q=2-31, 200
kb/s
Stream mapping:
  Stream #0.0 -> #0.0
[h263p @ 0xb7e7b650]removing common factors from framerate
Press [q] to stop encoding
frame=  150 q=3.8 Lsize=     322kB time=10.0 bitrate= 263.8kbits/s
video:322kB audio:0kB global headers:0kB muxing overhead 0.000000%

 
bash# ffmpeg -y -i foreman_raw_converted.rtp -vcodec rawvideo
foreman_raw_converted.avi
FFmpeg version CVS, Copyright (c) 2000-2004 Fabrice Bellard
  configuration:  --enable-shared --enable-static
  libavutil version: 49.0.0
  libavcodec version: 51.4.0
  libavformat version: 50.2.1
  built on Mar 27 2006 12:28:53, gcc: 3.3.3
Input #0, h263, from 'foreman_raw_converted.rtp':
  Duration: N/A, bitrate: N/A
  Stream #0.0, 15.00 fps(r): Video: h263, yuv420p, 176x144
Output #0, avi, to 'foreman_raw_converted.avi':
  Stream #0.0, 15.00 fps(c): Video: rawvideo, yuv420p, 176x144, q=2-31,
200 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
Press [q] to stop encoding
frame=   91 q=0.0 Lsize=    3385kB time=6.1 bitrate=4570.9kbits/s
video:3378kB audio:0kB global headers:0kB muxing overhead 0.196794%

 
as you may see, the duration between these steps is turned from 10
seconds (source sequence foreman.avi) to about 6.1 seconds in the
resulting foreman_raw_converted.avi. While playback, MPlayer tells me
that the frame rate of the resulting video is still 15fps, but the video
is played much faster than the original foreman.avi. 
 
Where is my mistake here? Or is it a bug?
 
Thanks a lot
 
greetings,
Hendrik