[FFmpeg-user] FPS mismatches after extracting video stream from avi file

[m.kamalasubha m.kamalasubha]

Hello all,
I was trying to extract video stream from an avi file using the command,

ffmpeg -i <input avi> -an -vcodec copy <output.avi>

*Input AVI file details:*
 Duration: 00:04:58.21, start: 0.000000, bitrate: 1312 kb/s
    Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p,
1280x720, 1054 kb/s, *24 fps*, 24 tbr, 90k tbn, 48 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz,
stereo, fltp, 253 kb/s (default)
    Metadata:
      handler_name    : SoundHandler

*Output AVI file details:*
  Duration: 00:04:58.08, start: 0.000000, bitrate: 1063 kb/s
    Stream #0:0: Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1280x720,
1054 kb/s, *48 fps*, 24 tbr, 48 tbn, 48 tbc

Why FPS differs between input and output avi ?