User blog comment:D. Cello/Assassinews 10/11 -- Desmond's Ancestry Revealed/@comment-4499860-20111012003917/@comment-98.250.36.251-20111012013015

I read it was because of the technology with the Animus. Now in Revelations Atair looks different because the Animus is updated, but it also can't make a difference between Altair and Ezio in the Black Room so Desmond looks like a combination of the two.