There are a number of reasons. Some of the stuff on Amazon is bought in bulk so the savings get passed on to the consumer. Individuals also sell through Amazon so that could be it. There are a number of reasons. You have to make sure that the copy isn't used or that you're getting Fritz 12 being sold as Fritz 13. I just went on Amazon and noticed that one company has a completely different DVD case image for Fritz 13.
fritz

I think that the Fritz on Amazon does not include the playchess subscription, but the software is otherwise the same.
I wrote a review on Fritz 13, that also has ideas for future enhancements, and some tips and tricks if you are interested: http://tacticstime.com/?p=1767
what is the difference between the fritz 13 that you can buy for less than £20 on amazon, and the fritz 13 that costs a lot more on chessbase?