I would suspect that the change was made because the original design would leave next to no space for stagehands to move between the walls.
Yes, I believe the exam table wall was usually closer to the shelf section of McCoy's office which could also explain why we didn't see more of that wall with the flat panel from the point of view in McCoy's office.
On the other hand, "Mirror, Mirror
" suggests that the arrangement here was very close to the original plans (with the exception of the white table that was possibly made of paper so the stuntmen wouldn't get hurt. A few scenes later when Sulu marches in we do have a sturdy black table there