Slackware 13 has new compression methode in its file with txz extention. It is a little bit different with tgz the earlier slackware used. It is because slackware 13 uses high more compression on its package compression.
Here is the news from distrowatch.com
Slackware, the oldest surviving Linux distribution, has made a small but important change to its packages – switching the compression from gzip to xz. Patrick Volkerding’s announcement in the change log says it all: “This batch of updates includes the newly released KDE 4.2.3, but more noticeably it marks the first departure from the use of gzip for compressing Slackware packages. Instead, we will be using xz, based on the LZMA compression algorithm. xz offers better compression than even bzip2, but still offers good extraction performance (about 3 times better than bzip2 and not much slower than gzip in our testing). Since support for bzip2 has long been requested, support for bzip2 and the original lzma format has also been added (why not?), but this is purely in the interest of completeness — we think most people will probably want to use either the original .tgz or the new .txz compression wrappers. The actual Slackware package format (which consists of the layout within the package envelope) has not changed, but this is the first support within Slackware’s package tools for using alternate compression algorithms.” Now that Slackware supports packages with different compression algorithms, it firmly puts the idea of having a standard suffix for all Slackware packages (such as “package.slk”) to bed.