The other vehicles it is being compared to are meant to go in harms way by their basic desgin. The Hummer is not. It is meant to take folks from point A to point B and not to go into active combat. Well, it surely wasn't designed to have explosives detonated under it. Although, in a small arms fight against infantry it should fare ok with the mounted machinegun, still NOT an assualt type vehicle. look at the picture of the M1A1 in the picture above this post.
You're right. It is a totally different animal when you're talking about driving over an IED that can contain several 500 pound bombs, as opposed to a conventional land mine. It is almost unbelievable that anything can do that to an M1A1.
Hell, you pile enough explosives together and you can blow away half a mountain.