It's like if source tarballs didn't include the compression algorithm in the extension but just used .tar for .tar.bz2, .tar.gz, etc. and just left the extractor to figure it out (which they can do).
It's more like .zip (which technically supports more compression algorithms and not just deflate) as the outer container format is the same for both Vorbis in .ogg and OPUS in .ogg (more commonly with the .opus extension) unlike tarballs where the archive format is wrapped by the compression format so a .tar.bz2 and a .tar.gz look nothing alike until you remove the compression.
