.avi means nothing, its just a container (audio video interlace) inside it has an audio and a video, these are then encoded using different methods of encoding (divx, xvid, x264, mp3, ogg, etc) the files are probably encoded differently, you have the codec for the first files, just not this one.
here is a list of the different container formats:
http://en.wikipedia.org/wiki/Compari...tainer_formats