You're wrong. Old iPods (e.g.: iPod Mini) use versions 2.x and their updates are for 2.x . The new iPods (e.g.: iPod nano 1st and 2nd Generations, iPod Fifth Generation) use versions 1.x and their updates are for 1.x . The fact is that 1.x versions are better than 2.x versions. These new versions start with a "1" because they are a new begining (Apple New Generations iPods). This is the explaination.
However, Ok... I can't find differences from version 1.3 to version 1.3.1
They know what they do.