Sounds like you are making the tars on Linux, then emailing from Windows?
This is generally a bad idea, especially if the receiver is a Windows user too - Windows needs third party software to handle tars (and tar.gz specifically) and most Windows users won't have an idea what to make such files.
It might be a variation of what another poster suggested, maybe the receiving mailclient tries to be clever and seeing the "strange" extension, renames it automatically?
The kicker is if the file, when called *_tar.zip, IS, in fact, a valid .zip file? I'm willing to bet it may be called "*_tar.zip" but that it still is a tar file...