I am not entirely sure how the calculations are done in order to determine the information contained in the genome of HIV, but my suspicion is that the reason this is more dense is due to the genome containing more than binary bits. I'm guessing that in the genome, for every atom that could be at a certain location in the genome, that would represent a different information state. So each location corresponds to more than a binary possibility (atom A, atom B, atom C, etc.). I think this is not the current goal as our purposes with memory storage have more constraints that merely density, what might increase density significantly could potentially decrease read/write speeds or maybe even energy required to perform read/write operations.

All of this is speculation and difficult for me to say for certain since I am not intimately knowledgeable on the topic, but that is my general guess.