Salut Tarara, it is Tiger Tree Hasing or TTH in action. In short, if a part of the download is corrupted, Gnucleus will know it before the file is totally downloaded and it will attempt to download the fragment of the file again.
Also please update your version of Gnucleus to the new builds that are found there;
http://gnucleus.com/betagnuc/index.php
These new builds might reduce the problem you noticed.
Bonne chance