Torrent swarms need at least one connectable (port forwarded) peer for any torrent data to transfer. In large torrent swarms you won't notice this too much since there are usually plenty of connectable peers available.
The effect tends to be more noticeable in smaller swarms.
In practice you may not notice unless you try to download those torrents with one lone seed who also happen to have no port forward. In those cases you'll see there's a seed but no torrent data ever transfers over to you. (note that the same happens in reverse if you're the lone seed on many torrents)