CauseThere is a special software which allows to transfer content of iPod Library to Windows which requires synchronization between iPod and PC.
iPod, iPod nano, and iPod mini can be restored to work with Macintosh or Microsoft Windows computers. If configured for a Mac, the iPod will be restored as Mac OS Extended (HFS Plus). If configured for Windows, the iPod will be restored as FAT32. Formatting happens automatically when you first set up your iPod. If for the first time you used your iPod with a Mac, during this process it became Mac formatted and vice versa.
You can check which format has your iPod using the Apple support article given below:
That is why you can not transfer information from the same iPod both in Macintosh and Windows. Once you synchronize iPod with iTunes library on Mac, the synchronization on Windows will fail.