Data to be written on the tape is compressed before its written. If the data is highly repetitive, it will have high compression ratio ( a Text file will be compressabile, 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 , ormore than the compressed capacity depending on how compressible your data is.