[FFmpeg-user] ffmpeg doesn't recognize the correct X resolusion when using x11grab

[Xeslaro]

  i'm currently running under resolusion 1366x768 as i can see from the output of xrandr:
Screen 0: minimum 320 x 200, current 1366 x 768, maximum 8192 x 8192
LVDS1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
   1366x768       60.0*+
   1024x768       60.0
   800x600        60.3     56.2
   640x480        59.9
VGA1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)

  however, ffmpeg thinks that i'm under 640x480, ffmpeg -f x11grab -i :0 outputs:
ffmpeg version N-53112-gd686517 Copyright (c) 2000-2013 the FFmpeg developers
  built on May 16 2013 09:15:01 with gcc 4.6.4 (GCC)
  configuration: --prefix=/home/xxx/p --enable-x11grab --enable-gpl
  libavutil      52. 32.100 / 52. 32.100
  libavcodec     55.  9.100 / 55.  9.100
  libavformat    55.  7.100 / 55.  7.100
  libavdevice    55.  0.100 / 55.  0.100
  libavfilter     3. 68.101 /  3. 68.101
  libswscale      2.  3.100 /  2.  3.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  3.100 / 52.  3.100
[x11grab @ 0x9bf7e00] device: :0 -> display: :0 x: 0 y: 0 width: 640 height: 480
[x11grab @ 0x9bf7e00] shared memory extension found
Input #0, x11grab, from ':0':
  Duration: N/A, start: 1368721262.140415, bitrate: 294617 kb/s
    Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 640x480, 294617 kb/s, 29.97 tbr, 1000k tbn, 29.97 tbc
At least one output file must be specified

  i'm running linux 3.9.0 with intel hd4000 graphic card, and the latest ffmpeg from git, and the 7.7 x.org release.
  anyone knows why this happens?