Did you notice that the first scan measured L1 - L4, but the second scan measured T12 - L3? The radiologist did not notice this. We managed to obtain the entire computer printouts to calculate L1-3 on each scan, and there was only a 1.1% difference, certainly within machine precision. The patient, who happened to also be a radiologist, was relieved to discover he had not lost bone density after all.
One reason this is important is that the vertebrae get denser as they get lower, so it is important to measure the same vertebrae each time.