That's what I see as well. I am still confounded looking at the renders. The existing office appears to be 4 of the large bays (you can see this on the amenity level) vs 2 large bays of additional office. That would be a 1/3 increase, minus the single missing floor. There is still a discrepancy of about 5 stories (of the original building floorplate) that are seemingly unaccounted for.
The floor plate of the existing building is ~639m2
The floor plate of the enlarged building is ~917m2
A gross enlargement of 43%.
Clearly some of the space is not being devoted to office.
The obvious bit is that there has to be a new/enlarged elevator core for all the new residential.
I'm not sure how how much of the space that accounts for, but it won't be immaterial. The additional elevators take up room (obviously) on every floor.
I imagine some material amount of space is also required for lots of additional plumbing stacks.
Typically in an office footprint of this site, you have them run to washrooms on every floor and a few break room/kitchettes.
There will be far more bathrooms and kitchens up top now, and far more plumbing runs accordingly.
But I'm not sure if that accounts for all of the difference (after adjusting for removing the one floor)