Upto LTO-5 the compression ratio was defined as 2:1. It assumed that 100GB data for example could be compressed to 50GB. From LTO-6 onwards the Compression ratio is defined as 2.5:1.
Compression ratio always depends on the data you are writing. A tar file can’t be further compressed for example, whereas a file of repetitive data can be compressed beyond 3 or 4 times.