Damage done to player controlled objects (planes, gv's) is calculated completely different than damage to OBJ such as buildings and ammo bunkers. As it stands now, the AP round does the exact same damage as the HE round from the Pzr IV, Panther, LVT-4, and T34/76. The AP round from the Tiger I and T34/85 do 114 lbs of damage (3 shots to bring down a building), so one would figure the King Tiger would be right in line.
EDIT: HOLD THE PRESS!!! I was just digging around for some further information on the HE rounds for the King Tiger and it appears that the AP rounds of the King Tiger were mainly the PzGr. 39/43 APCBC, those had a small charge of HE that needed about 30mm of armor penetration for ignition. I'm not sure of the TNT charge weight but if HTC modeled that in to their ammunition it would make sense as to why they have the King Tiger's AP rounds as effective vs OBJ as the HE round of the Panther, Panzer IV, etc etc.