rotation point of dts and dif objects outside the object bounding box?

I tried to build up a road/traffic environment with different dif and dts objects. While arranging the objects with translation and rotation input, i wonder why the objects do not rotate around their local origin/axis. The coordinate frame is shown in the geometric center of the object, but if a rotation is apllied to the object, it rotates around a point outside the object/bounding box.
Did anyone had similar problems or solved this problem?
Thank you very much.
Gregor