That's not typical, but can happen.

I'll usually see that at the end of a file, when the other sources finish and just one slow host is serving up the last chunk or so. By then, fast hosts are getting chunks you have already downloaded.

On larger files, it's useful to repeat the search, so new sources can be found when others have stopped. If downloading from 8 hosts, once the hosts drop below 8, repeat the search to find new sources (assuming more are available).

Not entirely necessary, but it can help when the downloads to you look to be stalling but the uploads are still going great.