When using qBitTorrent to download Linux ISOs, I am seeing that the total size of the download is one thing, then the total downloaded when complete exceeds (sometimes by almost double) that size.
It's probably because of the way torrent works. If some of the data from a peer doesn't pass whatever verification checks then it downloads it again. To me those look like normal variations in file size versus actually downloaded by the torrent client.
More retransmissions than overhead. This is big concern when downloading movies and TV shows because there are attempts to poison the DLs for people (or there was when torrent was first created as a protocol).