If I had to guess, it's because the way you read DNA is only along one of the strands (starting at the 5' end, IIRC), in which case you've still got four possib

If I had to guess, it's because the way you read DNA is only along one of the strands (starting at the 5' end, IIRC), in which case you've still got four possible "characters" to use in your representation- A-G is different from G-A, etc. People have thought up all sorts of clever ways to think of how one might encode data using DNA base pairs, but it's been too long since I did any bioinformatics for me to be able to remember exactly how they calculated it all out.