That sure doesn't sound right. I have a location running an LTO1, using LTO tapes and consistently is getting 80gb of 97gb max. I have read on this forum of folks upgrading to 10d and having to use a lot more tapes, but that was not my experience. On our upgrade from 9.1 to 10.d, the tapes stayed about the same, but to be honest, we don't have a need for a lot of multiple tapes, so....
When you say you are duplicating your backup from disk, do you mean that you are doing a backup to disk, then backing up that backup out to tape? If so, have you tried backing up directly to tape to see if you get the same result?