This is a subtle one. Did you notice that the total area for the first scan was 40 cm2, and for the second scan it was 76 cm2? But the images are to the same scale, and they look like they are the same size. When bone density is very low (0.498 g/cm2), some of the pixels do not contain enough mineral to be recognized as bone by the DEXA machines. These areas can be seen by the technologist, since the vertebra will have "holes" in it. The technologist for the first scan ignored these places, but the one doing the second scan forced the DEXA program to count the areas as bone. Therefore, the mineral content for each vertebra increased, but overall the bone density appeared lower because the new areas that were included in the second measurement had lower density.
Sometimes the bone area will appear larger because the bone mineral content is actually increasing, and more pixels will cross the threshold and be counted as bone. But they still are not as dense as the rest of the bone, so overall the bone density will decrease even though the patient is getting better. This might also have occured with this patient.
Meanwhile the hip, which has a more regular shape, showed the increase in bone density that is typically seen with bisphoshonates.