The Wikipedia page for TAR you posted in the answer says: "But today there are a number of add-on utilities which implement tar file indexing, thus enabling random access, both for raw tar files and for tar files compressed with gzip (which is amenable to indexing). Such an index can be kept in a separate file, appended or prepended to the archive file." I think that is what happens for example when using TAR through Dolphin/Ark instead of inside the command line.