Good question. I've read a bunch of articles about it (haven't delved into the research papers), and none of what I've read ever mentions that.

That said, I suspect it's based on the location and composition of the cave and it's surroundings.

I would like to know this too! I'm trying to think it must be some radioisotope dating, but I don't see what they would be comparing or sampling. It would also be interesting to do some DNA dating differential method between species.