I tested this quickly with a "house" class object and then with a normal object ("Land_Panelak" and "RoadCone" respectively) in the editor, and indeed you are correct : a createvehicle'd object such as RoadCone or M113 will be placed outside the building if you attempt to createvehicle it inside it. However, the house-classed object was happily placed inside the other house with no problems whatsoever. I also tried with an AmmoBoxWest, which behaved like the RoadCone. My only theory is that there's some difference in the configs of your particular box - are you sure it's not inheriting anything from the House classes?
Wolfrug out.