Im not sure if this is the case. I am not aware of any evidence besides that they didnt find two of the same bone at the site, that suggests that ALL of the bones were from ONE individual.
tell me you don't think they come wrapped in a pretty little package that says "lucy" or "linus"?
generally, bones are found in close proximity to each other. and if they are consisten in size, shape, and abuse level, they are considered to be tha same ndividual.
imagine if you lay out a small dead rat and a large dead rat for a couple years. they're going to rot and wash clean eventually. the wind, the rain, other animals, etc are going to move them around a bit. but you'll still have a dead small rat and a big rat relatively in proximity to themselves. but even after just a couple years, they probably won't be laid out like you see those pretty frescos of dinosaurs all laid out like a perfect xray. almost never happens.