Maybe the brain weighted 1200g and they misread the "2" as a "5" and put it down as 1500g. That would seem to make more sense.
When it comes to critical medical documentation that sounds pretty friggin stupid.
Matters not. Dr Cyril Wecht tried to obtain from the Archives... x-rays, slides, tissue sections and evidence of the remaining brain [what was left of it] and this was all missing with no explanation where it all went.
How come the doctors in Parkland didn't see the above, massive wound in the top of the President's head?
Witnesses who saw the head wound----
https://www.patspeer.com/chapter-18c-reason-to-doubt