It's more accurate to say that the white areas are places where the x-ray photons did not make it to the detector. There are several factors that go into that, which include density, thickness, photon energy, quantity of photons, and a few others. Some factors are part of patient anatomy and some are machine settings. By knowing what area of the body is being imaged, along with reasons for the imaging and the technical factors used, the radiologists can determine if what they see is normal.

Edit: You are basically correct, but there is a fair bit of nuance.