When you try to compress highly compressed or random data the size expands.
At least on the LTO tape drives I have used, will disable compression if the size is larger in an adaptive way.
As tape read and write speeds depend on data size, it is still worth the effort to try and opportunistically compress data on drive.
As this can usually be done without stopping or slowing the tape, there really isn’t much of a downside.
As for the compressed capacity, that is just 30+ years of marketing conventions, which people just ignore as it has always assumed your data was 2:1 compressible.
2.5:1 now apparently, showing my age because I had to go look because last time I had anything to do with LTO it was still 2:1 - guess they got PiedPiper to update the SLDC spec ;).