The values expressed for Tape usage are wildly inaccurate, and probably should not be trusted
When you format a tape, you format it to a specific value. In the Tape Cartridge box, there is usually a card that will give you the rated Compressed and Uncompressed values. Compression is handled on the Tape drive side
You format a tape to what you think it will hold, that is, what is on the card. If you use the default value, then this is what DP thinks the tape will hold, and this is considered the 100% mark
However, except for one case, this value is pretty much ignored, because DP will write to a tape until it is full, and reaches a Physical End or Tape marker. This becomes the new 100%. After protection has expired, and the tape is reused, capacity will be treated as the total amount of data last written to the tape
The one difference is when DP calculates if it has enough free media to complete the backup. If, for example, you format a 1.6TB tape to 400GB, and you need to backup 500GB (assuming that there are no other tapes avalable), you will get an error, and the backup wil not run until the tape is either reformatted to the correct value, or another tape is formatted, even though it will not get written to in this backup
Another factor in Tape Capacity is what type of Data is being backed up. You will fit a lot more data if you have a large number of small files, than a small number of large files. Also, some data compresses much easier than others.