It seems to happen rather often, so I have been wondering about that as well. Since I have always looked very closely I don't think it is just too well hidden to find the pointed out collectible.
I came up with this idea: the chief randomly points out a rare collectible and often enough You end up with one You have already found before. But tablet pieces never show up twice - they are unique. So maybe, if the chief still "chooses" a rare tablet piece to show up from an unseen list that has already been obtained before, You are simply unlucky and nothing will show up that time. I think that would explain why it keeps happening that You loose a chance to get a rare item.
But maybe I am all wrong and it is some mystical magical/natural reason behind this and You would only know, if the Chief's robe would fit You.