You have noticed a statistical fact. The spine has more trabecular bone and is more vulnerable to early loss. Vertebral segments are shorter subject to more force that is less distributed, and more subject to movement.
Hips are more likely to lose bone over the long period of uneven impact because of slight differences in leg length. In addition, with age comes increased possibility of other affecting diseases, like cancer. In my case, radiation through the hips has altered the bone marrow affecting the bone density. Cancers of the reproductive and digestive systems often cause bone loss through radiation treatment.
Sophie, you are interested in every detail. Your curoisity is wonderful. You even researched this.