>>63916425>Wait a minute, what?
Arkhan Land is talking with the Emperor. What he sees is a consummate, sterile scientist, with no emotion or feeling. This coincidentally reflects how Land sees a perfect member of the Mechanicum to be. The Emperor refers to Angron, who he is studying in the scene, as the Twelfth rather than as Angron.
However. While Land is listening to the Emperor talk, and discussing things with him, he notices weird things. Specifically, that the way the Emperor visually seems to be moving his mouth to speak doesn't register as speech, and because Land has augmentations that monitor things, that grants him the curious revelation that his auditory implants are not detecting words that either match with how the Emperor's mouth is moving, or with the meaning of them that he understands as he listens to the 'words' being spoken.
In brief, while Land is hearing (or rather, he thinks that he is, although he later realises that he actually wasn't) the Emperor describing Angron as the Twelfth, that is not actually what the Emperor is saying, or indeed ever said at all. It was always just Land's mind interpreting what the Emperor was saying in a way that he finds personally agreeable: the Primarchs as tools of the Omnissiah, and not beings in their own right. That is Land's belief, and so when the Emperor speaks to him, that's what he interprets, hence he hears Twelfth instead of Angron's name.
It's all over the place: the psychic aura of the Emperor means that nobody is seeing or hearing the real him except for the Sisters of Silence (who describe him as generally nondescript but ageless, with shoulder-length dark hair). Corax also briefly saw through the aura at one point, and similarly saw that same thing, an ageless but nondescript dark-haired man, with multi-coloured heterochromatic eyes.