Ok ;et me explain this, the Statistics window shows your overall download, ie data downlaoded/uploaded on all torrents till date, mine shows Downloaded 90 GB, that by no means is what I downlaoded last night, isn't it
The Summary window shows what you are downlaoding/uplaoding for that torrent. Now you say you have downloaded 849.46 MB for a 793.54 MB torrent, that is perfectly normal. (Quote Sushubh's Post No. 32 here)
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Now Bitcomet is known for such errors, the best way to reduce these errors is by going to Preferences-->Advanced-->Connections and reduce the Connections to keep per task and Maximum connections per task to around 10, besides many people on Public trackers use Azu MODS etc further increasing errors. My friend used to download twice the data on each torrent even from private trackers, I asked him to reduce the no. of connections and now he downloads around 10-15 mb max extra data which doesn't appear in rubbish data dropped (use Total downlaoded + remaining - Size of torrent = Extra data downloaded)
If you do not wish to download that too, then use some other client like Azureus or BitTornado, I never used them because I'm really used to BitComet now so switching to some other client will be a pain
. If you have a ot of CPU and RAM I'd suggest you go for Azureus, BitTornado on the other hand has no queues, every torrent appears in its own window and its a little too messy IMO.