They are not dating the bone, they are dating the mud around the bone. To be more precise, they are dating the last time the mud around the bone received sunlight. (And from the patterns in the mud they are sure that nobody dig a hole at midnight to put the bone there.)

Well probably not as a joke, but it's within the realm of possibility that it was moved there at some point in the past. This is pure speculation of course, but perhaps it was identified as a sacred object and carried there by a traveller sometime within the last couple of thousand years. However the likelihood of that is probably quite slim.

  "as a joke"

There's no need to project motive onto such a premise. One needs only to question the possibility of movement.

Religious rituals are just as likely, as well as being deposited with ani al droppings after becoming an unfortunate snack for some scavenger or carnivore. Also flood runoff, why not?