I don’t know the internal workings of IA and the bittorent architecture but if an archive has too many items the torrent file won’t have them all. I see this all the time with ROM packs and magazine archives for example. +1000 items, the torrent will only have the first ~200 or so available

I think for some reason IA limits the torrent size. I have seen as low as 50 with a 1000+ item archive.

Even better, try this:

    btplay https://archive.org/download/BigBuckBunny_124/BigBuckBunny_124_archive.torrent

What is that tool?

I guess this: https://github.com/johang/btfs/blob/master/scripts/btplay