Check for the tab which says Hashfails/Wasted Data ( if it is there in the details).
Could be possible that extra download is due to either hash fails or discarded/wasted data.
Figure mentioned under the Hashfail/Wasted data might explain the extra download you see in the download client.
**Discarded data is the unwanted data sent to you by a peer whereas Wasted data is a combination of discarded data and bad data.
** More elaborately, Every piece you download is checked against a hash. If the hash fails verification, the data is discarded and requested for download again. The 'Hash Fails' tab shows the number of hash fails in terms of MB/GB.
Basically seems like client is throwing out either unwanted data or data which it is not able to verify against.