That bothered me too, in my head canon Sarek's in an identical room and was actually leaning on /his/ desk.
But explaining the unVulcanlike posture is another matter all together...
Seeing as the holograms jump location from time to time, (even without the interference), I would say it is predictive, and when the computer saw that he was about to sit down, it placed him in a convenient location within Burnham's quarters.