Data to be written on the tape is compressed before its written. If the data is highly repetitive, it will have a high compression ratio ( a Text file will be compressible, a BMP image is highly compressible). But if the data is already compressed ( say you have tarred or Zipped the data already), it won't be further compressible.
For Data That's non-compressed you will see capacity higher than the native capacity. For data that's already compressed, you will capacity close to the native capacity.
TLDR: Native capacity is what is guaranteed. You might get Compressed capacity or more than the compressed capacity depending on how compressible your data is.