There could be several factors involved in why that one is white vs dark colored. (sorry if someone already answered this one, as I read more into the thread, I will probably see similar answers!)
1) a dry sunny area - sun has a bleaching effect
2) it was not submerged under water during decomp...damp, moist, shady areas cause an abundance of molds & algae to grow on the surface, darkening it.
3) due to the presence of water in the case of Caylee's remains, apparently the bleaching process did not occur, although mostly skeleton was left, IIRC the skull still had some hair attached to it which leads me to believe possibly dried tissue matter as well. As you can see from the picture of the deer skull, no evidence of skin, hair or anything remain on the surface. Apparently, the deer skull was not contained in a plastic bag, either, so there was no shelter between that skull and the elements.
This is my opinion only! Being raised in the country and the woods, I've seen the remains of various wildlife, the conditions of the bones are typically situational, definitely damp shady areas are not conducive to bleached white bones.