Why cremated corpse turn into powder when cremated when in fact bones are incombustible?

Question

From what I know, bones are incombustible. But in all cases of cremations I know after-cremation-leftovers are placed in a jar-like containers called urn.

Answer 1

A complete cremation is a two-step process. Firstly, the actual exposure of the deceased to several hours of intense heat and flame; after which the remains are mostly ash except for certain bone fragments, then the entire remaining ash and fragment volume is gathered and run through a processor, creating a uniform powder-like texture.
http://www.icfa.org/cremation.htm#Cremation1
http://en.wikipedia.org/wiki/Cremation

Answer 2

Bones are not incombustible. They are made up of a cartilage-like matrix infiltrated with calcium salts. If the matrix is destroyed, only the loose calcium salts remain, in the form of a "powder".

Answer 3

I believe everything that makes up the human body will burn. Now if someone had a titanium hip replacement it probably wouldn't burn, but bones definitely burn.