The original report noted that the skeleton was well preserved for its age.

But even well-preserved collagen can carry the freshwater reservoir problem when the diet of the individual included substantial freshwater fish.

This is not contamination in the conventional sense — there is no external carbon that seeped into the sample after burial.

It is a property of the carbon that was already in the bone when the individual was alive.

The fish she ate contributed their own radiocarbon signature to her body, and that signature was systematically depleted in carbon-14 relative to what the atmosphere would have provided through a purely terrestrial diet.

This is why the problem cannot be solved by better pretreatment alone.