Some tissue and some areas of the body decompose more quickly than other areas. The face is one such area, mainly because it is actually not very "deep" - it overlies the bones of the facial skeleton very closely. Another area where that occurs is on the front of the shin, overlying the tibia - and could explain the defect found there in the autopsy. It could have been simply the skin and tissues "tearing" away from the underlying bone.
The face exhibits similar characteristics - there is a fat pad overlying the cheek area beside the mouth in most people, but apart from that, there's not a lot of other tissue apart from a thin layer of muscles that are responsible for facial expression.
And of course, if the body was lying in a position where the high tide partially covered some parts of it, then those parts would have been decomposing in an accelerated fashion, too.
So, from the position she was found to be in, it is conceivable that at least one of her legs, and perhaps her face too, could have been partially submerged on the high tides - which is why Nathan Milne wanted to get her up and away from her resting position a.s.a.p. as the high tide was approaching. That was stated in court, as well as in the autopsy report.
Hope that makes sense....
Will be back soon.
