Why didn’t the Nobel Assembly give the prize to Gallo?

Today’s most interesting medical history news is not that Françoise Barré-Sinoussi and Luc Montagnier have been awarded half of this year’s Nobel Prize in Physiology or Medicine for “their discovery of human immunodeficiency virus”. The interesting news is that Robert Gallo doesn’t share the prize, and that he is not even mentioned in the press release (only in the technical appendix).
In hindsight, both scientists and historians probably agree that the French group made the actual discovery of the virus which was later named HIV. Yet, Gallo and his group at NIH played a significant role both before and after the actual discovery, especially in determing the causative role of HIV for the development of AIDS. Asked about his reaction to the news, Gallo reportedly says it was “a disappointment” not to be included.
I guess it’s very much a question of what is meant by ‘discovery’ and also which discovery we are talking about. Nobel’s will emphasized ‘discoveries’ (Swedish: ‘upptäckter’), which speaks in favour of awarding Barré-Sinoussi and Montaigner alone. On the other hand, the Nobel Assembly at Karolinska Institutet could have chosen to award the establishment of HIV as a causative agent of AIDS rather than the discovery of the virus itself — in that case Gallo would probably not have been excluded (but then again Harald zur Hausen would not have been awarded for “his discovery of human papilloma viruses causing cervical cancer”, since only three people can be awarded each year).