daisy17, I see how that sentence is completely unclear.
The premise evolves from the way bisphosphonates work. They don't add new bone. They preserve older fissured bone, increasing bmd by accumulation. The amount of mineral in a measured space doesn't reveal the structure of the minerals. The bones we save with bisphosphonates are just as porous ( while increasingly brittle and avascular). In the same technical sense that a crushed vertebra reads as highly mineralized, areal mineral density increases without improving porosity.
Before 2002 bisphosphonates were all we had to slow down bone processes. I think it is true that bisphosphonates have prevented some fractures. But maybe more fractures occur among women who are osteopenic than among those who are osteoporotic because the dexa-definition of osteoporosis is disguised by use of bisphosphonates.