Human bone density and Brown Bear bone density?
The word density in the medical term bone density is used to quantify only the mineral content of the bone (bone mineral). It's not intended to describe the total mass density of the bone like we are used to from physics.
This is all on wikipedia: http://en.wikipedia.org/wiki/Bone_density
Thus, when one say that a Grizzly bear have 10 times the bone density, it implies that its bones have 10 times the mineral content compared to humans, and this is indeed seen as a proxy indicator for bone strength. However, this claim does seem unsupported and unrealistic as kmm points out.