What type of the job you're using? Backup to tape or files to tape?
If that's the case, then we wouldn't be having this problem, so why is it backing up more than that?
In fact, we receive information regarding tape free space from tape device directly. Your device must have notified us that the tape medium was full and we took another cassette. Depending on manufacturer, the tape might not be necessarily 100% full, when further write operation is prohibited, and there might be some free space left
I submitted a case, but never received a case number.
You can open a support case and provide required logs by going to http://cp.veeam.com/