I've tested it at Windows7 64 bits:
- Corrupt file according to Winrar and only two files inside (neither libadobe nor librmsdk were included).
- OK file according to 7zip (extraction in two steps) and all the files inside
- Tar file ok according to Winrar after extracting the tar from the tgz with 7zip
So, perhaps we can think that the problem is not the tar itself but the tgz (the compression).
Additional info: The initial KoboRoot.tar.gz has a size of 14.614.52 bytes, and after using gunzip and gzip (I have cygwin installed in my computer), the size is 14.311.424 bytes and Winrar doesn't complain about the file.