I wish I had an answer for you.
HQ hardness is 37,500 lbs. If a convoy from the City was just outside the HQ when you dropped on it, it could have resupplied it before the last formation dropped. Did every bomb hit the building? Films and anecdotal evidence or impressions are very different sometimes.
The other question is about the cookie bomb. I haven't tested it yet. The 2,000 lb. and 4,000 lb. bombs are not actually 2k or 4k of ordnance. The 2,000 lb. bomb is equivalent to only 1,718 lbs. of ordnance. I'll try to test to cookie and see what I come up with.