A tar.gz is a tar (tape archive, a type of file in which multiple files are stored), compressed with gzip, a compression system. There are a number of compression systems used with tar, but gzip is the most common. A plain tar file is not compressed.
So, that's the background. Now, when you download a compressed archive, Apple's Safari browser will very helpfully decompress it for you. So nice of it! There's a catch, though. For zip files, it's fine. For tar.gz files, however, it ungzips it... but doesn't untar it. I can't really think of any reasonable situation where a tar would be preferable to a tar.gz; it just sits there using extra disk space. They should either have left well enough along, or uncompressed and unarchived it.
I suspect that, one day in the early noughties, someone in Apple was handed a task labelled 'implement decompression in Safari downloads' and took it way too literally. This isn't some new bug, by the way; it's been that way since at least 2005.