VLC is able to start playback of a currently downloading file and play arriving data.
SMPlayer can't do that. You have to restart playback to be able to see new file data.
Really useful feature - starting video download and being able to play right off the bat without waiting for complete download.
PS. I think it's because (S)Mplayer doesn't account the filesystem state of files it opens. On file opening the (S)Mplayer inappropriately fixate the end of file for files that are still being written at.