FFMPEG: extract images from video with fixed interval, first few frames messed up -

i wanted extract frames video fixed interval, used command lines like:

ffmpeg.exe -i input.mp4 -r 5 results\%d.jpg` 

however, interval between first few frames different of rest.

for example, if both ffmpeg.exe -i input.mp4 -r 25 -t 5 test\%d.jpg , ffmpeg.exe -i input.mp4 -r 5 -t 5 test2\%d.jpg (for fps 25 video), expect results in test2 subsequence of results in test interval 5. got are:
screenshot of results

from thumbnails, obvious images 112~125 in test not appear in test2. close inspection reveals correspondence between 2 set of results follows:

test  | test2   1&2 |   1&2 (those 2 frames identical)   3   |   3   4   |   4   9   |   5  14   |   6  19   |   7 ...   | ... 104   |  24 109   |  25 

(actually, when comparing last frames, realized results in test had mosaic. fixed adding option -q:v 1.)

as can seen, first few frames test2 have interval 1, instead of expected value 5.

what reason behavior, , how fix it?

in case want see ffmpeg outputs:

ps c:\users\<username>\documents\new folder> ffmpeg.exe -i input.mp4 -r 25 -t 5 test\%d.jpg ffmpeg version n-50911-g9efcfbe copyright (c) 2000-2013 ffmpeg developers   built on mar 13 2013 21:26:48 gcc 4.7.2 (gcc)   configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfi g --enable-frei0r --enable-gnutls --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetype --enable-libg sm --enable-libilbc --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --ena ble-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-lib twolame --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxa vs --enable-libxvid --enable-zlib   libavutil      52. 19.100 / 52. 19.100   libavcodec     55.  0.100 / 55.  0.100   libavformat    55.  0.100 / 55.  0.100   libavdevice    54.  4.100 / 54.  4.100   libavfilter     3. 45.103 /  3. 45.103   libswscale      2.  2.100 /  2.  2.100   libswresample   0. 17.102 /  0. 17.102   libpostproc    52.  2.100 / 52.  2.100 input #0, mov,mp4,m4a,3gp,3g2,mj2, 'input.mp4':   metadata:     major_brand     : mp42     minor_version   : 0     compatible_brands: isommp42     creation_time   : 2015-05-05 04:46:16   duration: 00:49:04.96, start: 0.000000, bitrate: 386 kb/s     stream #0:0(und): video: h264 (constrained baseline) (avc1 / 0x31637661), yuv420p, 524x360 [sar 1:1 dar 131:90], 287  kb/s, 25 fps, 25 tbr, 25 tbn, 50 tbc     metadata:       handler_name    : videohandler     stream #0:1(und): audio: aac (mp4a / 0x6134706d), 44100 hz, stereo, fltp, 96 kb/s     metadata:       creation_time   : 2015-05-05 04:46:21       handler_name    : isomedia file produced google, 5-11-2011 output #0, image2, 'test\%d.jpg':   metadata:     major_brand     : mp42     minor_version   : 0     compatible_brands: isommp42     encoder         : lavf55.0.100     stream #0:0(und): video: mjpeg, yuvj420p, 524x360 [sar 1:1 dar 131:90], q=2-31, 200 kb/s, 90k tbn, 25 tbc     metadata:       handler_name    : videohandler stream mapping:   stream #0:0 -> #0:0 (h264 -> mjpeg) press [q] stop, [?] frame=  125 fps=0.0 q=0.0 lsize=n/a time=00:00:05.00 bitrate=n/a video:1140kb audio:0kb subtitle:0 global headers:0kb muxing overhead -100.001884% ps c:\users\<username>\documents\new folder> ffmpeg.exe -i input.mp4 -r 5 -t 5 test2\%d.jpg ffmpeg version n-50911-g9efcfbe copyright (c) 2000-2013 ffmpeg developers   built on mar 13 2013 21:26:48 gcc 4.7.2 (gcc)   configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfi g --enable-frei0r --enable-gnutls --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetype --enable-libg sm --enable-libilbc --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --ena ble-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-lib twolame --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxa vs --enable-libxvid --enable-zlib   libavutil      52. 19.100 / 52. 19.100   libavcodec     55.  0.100 / 55.  0.100   libavformat    55.  0.100 / 55.  0.100   libavdevice    54.  4.100 / 54.  4.100   libavfilter     3. 45.103 /  3. 45.103   libswscale      2.  2.100 /  2.  2.100   libswresample   0. 17.102 /  0. 17.102   libpostproc    52.  2.100 / 52.  2.100 input #0, mov,mp4,m4a,3gp,3g2,mj2, 'input.mp4':   metadata:     major_brand     : mp42     minor_version   : 0     compatible_brands: isommp42     creation_time   : 2015-05-05 04:46:16   duration: 00:49:04.96, start: 0.000000, bitrate: 386 kb/s     stream #0:0(und): video: h264 (constrained baseline) (avc1 / 0x31637661), yuv420p, 524x360 [sar 1:1 dar 131:90], 287  kb/s, 25 fps, 25 tbr, 25 tbn, 50 tbc     metadata:       handler_name    : videohandler     stream #0:1(und): audio: aac (mp4a / 0x6134706d), 44100 hz, stereo, fltp, 96 kb/s     metadata:       creation_time   : 2015-05-05 04:46:21       handler_name    : isomedia file produced google, 5-11-2011 output #0, image2, 'test2\%d.jpg':   metadata:     major_brand     : mp42     minor_version   : 0     compatible_brands: isommp42     encoder         : lavf55.0.100     stream #0:0(und): video: mjpeg, yuvj420p, 524x360 [sar 1:1 dar 131:90], q=2-31, 200 kb/s, 90k tbn, 5 tbc     metadata:       handler_name    : videohandler stream mapping:   stream #0:0 -> #0:0 (h264 -> mjpeg) press [q] stop, [?] frame=   25 fps=0.0 q=0.0 lsize=n/a time=00:00:05.00 bitrate=n/a dup=0 drop=88 video:375kb audio:0kb subtitle:0 global headers:0kb muxing overhead -100.005728%