One question.

I have a torrent, in Tixati it shows 12,3 GB. The same torrent in BitTorrent and uTorrent shows 11,5 GB. Why is there such a discrepancy?

While most torrent clients are using tebibytes, gibibytes, mebibytes, kibibytes, Tixati by default measures the data using terabytes, gigabytes, megabytes, kilobytes.
1 gigabyte(GB) = 10^9 bytes = 1 000 000 000 bytes // 1 gibibyte(GiB) = 2^30 bytes = 1 073 741 824 bytes. You can change the units used by Tixati this way: Click on the gear icon (top right corner) > Output formatting, then change largest unit for bytes, bytes in charts, numbers, numbers in charts to Ti or Gi or Mi or Ki