do you mean the jenova formerly in the shinra tower or the one you fight?

i believe that the one in the tower is the dead corpse of the jenova of the past while the one you fight is in fact some of the clones joined together...
eg. jenova birth is of the less important clones
jenova life is average clones
jenova death is more important
jenova absolute is most important with Sephiroth as the leader but not an actual clone and Cloud as a 'failed experment' therefore not joined with any of them... zack is dead so i dont count him...