how would anything be High Explosive without any explosive material to make it High Explosive???
<cough>fragmentation<cough>
*sorry, i just had to*
Legit question. However, how could anything with a pinch of explosive be termed "high" explosive?

Also, Looschy pointed out something very important that has been mentioned previously: the hardness setting are %100 arbitrary, there is no set standards for what OBJ gets what hardness value unless like X plane is able to travel Y fast at Z altitude. Everything in regards to OBJ can be adjusted.
From past experiences in watching Hitech and others from HTC respond to issues like this, I'd be willing to bet they have put all things OBJ (non hangers and SB) at one low setting to allow the broadest number of aircraft and gv's to be able to destroy the OBJ in question. Hence, why the F4F and a P51D are on "equal" terms in ability to gun down a radar tower, ammo bunker, building, etc. Sure, the P51D can bring along almost 3000 lbs of ordnance for a major advantage, but again we're talking about the lowest common denominator in having 312 lbs of damage needed for the typical OBJ to be destroyed.