This webm-file contains wrong timing information (or at least it parsed incorrectly by mkvtoolnix and ffms:
timecode file) according to which first 4312-frames have duration of 2:17.5 but in reality it is constant 30 fps video and this 4312-frames should have duration of 2:23.73. I guess this problem can have something todo with not-displaying frames of VP8 (which imho shouldn't present [ignored] in timecode file [parsing]).
Edit: that is ffms bug not lavf (because with --demuxer lavf it should result with correct duration)