Many vessels have been found with stones inside - some near the bow (as in this case), some amidships. There are plenty of theories - some exotic. I'd like to think that there is a very simple explanation, and consider that these people were just as smart as you or I, if not smarter.
We really know so little about them, and these stones serve to prove the point. We can't use our own values to assess them either - these guys thought very differently, and did things for reasons we can only guess at.