LTO tapes are written in a serpentine manner. Depending on the LTO generation , the tape moves between 44 times (LTO 3) and 136 times (LTO 6) between the beginning of tape to the end of tape and each time writes one "wrap^*", in alternating directions. A "full length overwrite" overwrites ONE of these wraps, which is between 2.3% (LTO 3) and 0.7% (LTO 6) of the data. The remainder of your data are still on tape, unchanged, untouched, and retrievable.
* A wrap consists of 8 data tracks (LTO1 and LTO2) and 16 tracks (LTO3 - LTO6).

Overwriting all data on tape takes a long time and is therefore expensive. The "solution" to this problem seems to be to just overwrite a small amount of the data and call it something that may sound like all data have been overwritten.